WASHINGTON – The plan by Senate Republican leaders to pass a scaled-back overhaul of the Affordable Care Act was endangered Thursday when key Republican senators refused to back the effort without assurances that it would become the basis of negotiations with the House.

The extraordinary spectacle in which an early evening news conference was convened by Sens. Lindsey Graham, R-S.C., John McCain, R-Ariz., and Ron Johnson, R-Wis., highlighted the extent to which Republicans are struggling to reconcile their desire to tear down the 2010 law known as Obamacare with their inability to unite behind a replacement.

The three senators – who collectively could sink the Republican plan – said their vote for the “skinny repeal” measure would not be used to trigger a hasty vote on such legislation by the House before it heads home for recess. Many conservatives in both chambers object to the measure that would remove key insurance mandates and make a handful of other changes because it does not go far enough in repealing the ACA.

“If I don’t get those assurances I’m a ‘no’ because I’m not going to vote for a pig in a poke,” Graham said, adding he would happily defend such a vote if he doesn’t get the guarantee he is looking for. “I do this with joy in my heart.”

The announcement came a few hours after Senate Majority Leader Mitch McConnell, R-Ky., sent an email to his conference outlining the bill now-dubbed “skinny repeal.”

It would eliminate the law’s requirement that Americans obtain coverage or pay a tax penalty, and suspend the mandate that firms employing 50 or more workers provide insurance for at least five years. The measure also would eliminate funds for preventive health care provided under the 2010 law and transfer the funds Planned Parenthood would receive for one year to community health centers. Finally, it would provides states more flexibility in how they administer their Medicaid programs under the law’s 1332 waiver program.

While the revamped health measure is more modest than earlier versions of the bill, it would still have a major impact on the individual insurance market. Eliminating the ACA’s individual mandate could change the makeup of those buying insurance, and cause the premiums of those remaining in the system to rise significantly.

Several Senate Republicans, however, framed it as just a vehicle to keep alive their seven-year long effort to repeal the ACA.

“My sense is people aren’t so much focused on the substance as they are this being the lifeline to get to a conference and expanding the bill,” said Sen. Bob Corker, R-Tenn.

And Sen. Rob Portman, R-Ohio, tweeted, “I will support legislation to move this process to a House-Senate conference because I believe we need to repeal and replace Obamacare.”

And while Senate Republicans across the political spectrum described the latest proposal as a way to sustain the ACA rollback effort, the prospect of an immediate up-or-down vote in the House raised alarms.

A Republican deputy whip who spoke on the condition of anonymity to discuss internal deliberations said there was a definite possibility that the House would pass any Senate bill intact. But the member said hard-line conservatives could balk, and send the bill to conference.

House leaders were reticent to commit publicly to a joint negotiation with the Senate. Ryan spokeswoman AshLee Strong said conference committee was “one option under consideration.”

As Senate Republicans sought to move their plan closer to passage, House Republican leaders instructed their members to not to leave town for their month-long summer recess just yet.

“Members are advised that – pending Senate action on healthcare – the House schedule is subject to change. All Members should remain flexible in their travel plans over the next few days,” said the office of House Majority Leader Kevin McCarthy, R-Calif. His announcement left open the possibility of a vote to create a conference committee – or perhaps on the skinny repeal itself.

But key House conservatives said they would not back “skinny repeal” if it came over from the Senate in its current form.

Rep. Mark Meadows, R-N.C., chair of the House Freedom Caucus, said he wouldn’t vote for “skinny repeal” and didn’t think other conservatives would, either.

“My nose would say that there’s enough that it would lack the support,” he told reporters.

Even if Republicans succeed in passing a bill this week – and start negotiations with the House – they will face significant obstacles in accomplishing anything more substantial.

Speaking on the Senate floor Thursday, McConnell emphasized that the votes this week would not reverse the law even if they culminate in passage of a bill. The votes are expected to be held through the night and into Friday morning.

“One phase of that process will end when the Senate concludes voting this week, but it will not signal the end of our work. Not yet,” McConnell said.

While McConnell predicted there would be an all-night marathon of votes, Democrats have decided to save their political ammunition until Republicans reveal the substance of the skinny repeal that they have been crafting.

Senate Minority Leader Charles Schumer, D-N.Y., said in a Senate floor speech that Democrats would offer no more amendments until that point, so Republicans could not claim that their final bill was the product of extensive Senate deliberations.

Schumer said that if the skinny repeal plan passes, the Senate should then be prepared for “numerous” Democratic amendments, which could potentially force Republicans into politically tough votes.

Top Republicans, such as Sen. John Thune, S.D., No. 3 in the chamber’s Republican hierarchy, said that although leaders have not yet found “the sweet spot” between conservatives and centrists, they have picked up support for a more modest plan because it does not include deep cuts to Medicaid. Some Republican senators were simply open to any legislation that could keep alive the roller-coaster push for an overhaul.

“We’re edging closer and closer” to getting 50 votes for a bare-bones plan, Thune said. He said leaders were betting that some Republicans who defected on votes this week would feel more pressure to support any bill that emerged from negotiations with the House to face a final vote in the Senate.

“Voting on something at the end of the process when it’s the only train leaving the station . . . I think that’s a different vote for a lot of people,” he said.

In an effort to muster enough votes for a narrow bill, Republican leaders suggested that even some proposals that have died in the Senate could come up again once they enter negotiations with the House. And some members have begun to try to add a few additional provisions to the skinny bill, using their leverage to try to strengthen their negotiating position in conference.

Sen. Rob Portman, R-Ohio, is pushing for the inclusion of $45 billion to treat opioid addiction, Republicans familiar with the talks said Thursday. It was unclear whether the funds would be included, given the bill’s budget constraints. Aides to Portman and McConnell did not immediately comment.

Portman and Sen. Shelley Moore Capito, R-W.Va., successfully lobbied leadership to include the money in the more sweeping repeal-and-replace bill that the Senate voted down earlier this week. But it was unclear whether they would prevail once again.

Graham said earlier in the day he was willing to go along with the leadership’s latest legislative approach to extend the negotiations in a House-Senate conference committee – but only if he received assurances that his ideas could be resurrected in that setting.

“I won’t vote for the skinny bill unless Graham-Cassidy is conference-able,” said Graham, referring to an alternate health bill he has offered with Sen. Bill Cassidy, R-La., that has gained little traction.

President Trump exhorted Republicans on Twitter on Wednesday morning to fall in line and back some sort of proposal to scale back the law known as Obamacare.

“Come on Republican Senators, you can do it on Healthcare,” he tweeted. “After 7 years, this is your chance to shine! Don’t let the American people down!”

While McConnell has led the negotiations over a health-care plan for weeks, Trump has sought to drum up support by publicly pressing wavering Republicans.

Interior Secretary Ryan Zinke called Alaska Republican Sens. Dan Sullivan and Lisa Murkowksi separately on Wednesday to warn them that the administration may change its position on several issues, according to individuals briefed on the conversations, given Murkowski’s vote against proceeding with health-care legislation this week.

Since Trump took office, Interior has indicated it is open to constructing a road through the Izembek National Wildlife Refuge and drilling in the Arctic National Wildlife Refuge while expanding energy exploration elsewhere in the state. But now these policy shifts may be in jeopardy.

Speaking to reporters Thursday, Sullivan said the Trump administration has previously been cooperative on Alaska issues with Murkowski, who chairs the Senate Energy and Natural Resources Committee.

“From my perspective, the sooner we can get back to that kind of cooperation between the administration and the chairman of the ENR committee, the better for Alaska and the better for the country,” he said. Sullivan said he is not telling Murkowski how to respond.

The Alaska Dispatch News first reported the calls.

Interior officials did not respond to a request for comment.

McConnell overcame serious opposition from his rank-and-file members to begin debate on health care – a prospect that seemed dim just last week. Trump has taken to Twitter and made public statements challenging Senate Republicans to support an overhaul or take ownership of the ACA’s failure.

But in two votes within 24 hours of each other, lawmakers rejected differing approaches to rewriting the landmark 2010 law. On Wednesday the Senate rejected a proposal that would have repealed major parts of the ACA outright, with seven Republicans joining all of the chamber’s Democrats in opposition. The day before, a replacement bill that would have weakened the law’s insurance rules and provided more money to help Medicaid recipients transition to private insurance failed by a slightly larger margin.

With the idea of a scaled-back bill emerging as the most likely outcome of this week’s votes, the proposal now faces increased scrutiny.

Senate Democrats announced late Wednesday afternoon that a preliminary CBO estimate found that 16 million people would lose coverage and that premiums would rise 20 percent if Republicans enacted a handful of the policies floated for the pared-down repeal bill. The analysis was based on the assumption that the Republican wants to repeal the individual and employer mandates, end a 2.3 percent tax on medical device manufacturers, ban funding for Planned Parenthood and repeal funds for preventive health care.

In a sign of how the prospect of a spike in the uninsured rate continues to worry governors, a bipartisan group of 10 of them – including Republicans Brian Sandoval of Nevada and John Kasich of Ohio – urged Senate leaders late Wednesday to work together with governors in developing a new plan and to reject a skinny repeal, which they said “is expected to accelerate health plans leaving the individual market, increase premiums, and result in fewer Americans having access to insurance.”

Meanwhile, the ongoing uncertainty on Capitol Hill sent jitters through the insurance industry.

Joseph R. Swedish, the chief executive of Anthem – the nation’s second-largest health insurer – said on a conference call to review second-quarter earnings that the company is reassessing its participation in ACA marketplaces for next year. Anthem has decided to largely withdraw from the markets in three of the 14 states it participates in, and he said it may stop participating elsewhere unless the markets seem stable.

The Blue Cross Blue Shield Association warned that even the skinny repeal Republicans now envision could undermine the individual insurance market because it would eliminate the individual mandate. The measure, which remains subject to negotiation, also would probably eliminate the ACA’s requirement that employers with 50 workers or more provide health coverage, and a medical device tax that generates $19.6 billion in federal revenue over a decade.

And America’s Health Insurance Plans, a major insurance trade association, weighed in with a letter Thursday saying it opposes “targeted proposals that would eliminate key elements of current law without new stabilizing solutions.”