The Republican Party was in a state of pandemonium Wednesday as a clutch of independent groups scrambled to throw together a last-ditch effort to deny Donald Trump the presidential nomination, even as some party figures concluded it is now too late to stop the billionaire mogul.

The decentralized and desperate stop-Trump campaign found a possible new leader in Mitt Romney, the party’s 2012 presidential nominee, who is expected to deliver a forceful, top-to-bottom indictment of Trump in a speech Thursday.

Top Republican donors and strategists laid plans for a multimillion-dollar assault on the front-runner in states holding contests on March 15. Ground zero is Florida, where home-state Sen. Marco Rubio, the leading establishment candidate, is going all in to defeat Trump, who leads in the polls.

But other Republicans yielded to Trump after he swept seven of 11 states on Tuesday, the biggest day of balloting yet. Alex Castellanos, a veteran media consultant who earlier in the season had tried unsuccessfully to organize an anti-Trump campaign, said, “A fantasy effort to stop Trump . . . exists only as the denial stage of grief.”

“Trump has earned the nomination,” Castellanos wrote in an email. “Donald Trump whipped the establishment and it is too late for the limp GOP establishment to ask their mommy to step in and rewrite the rules because they were humiliated for their impotence.”

Similarly, William J. Bennett, a Reagan education secretary, said he could not support the anti-Trump movement.

“I’m used to being the moral scold, but Trump is winning fair and square, so why should the nomination be grabbed from him?” asked Bennett, now a conservative radio host. “We’ve been trying to get white working-class people into the party for a long time. Now they’re here in huge numbers because of Trump and we’re going to alienate them? I don’t get it. Too many people are on their high horse.”

The deepening division in the party came as the field appeared poised to narrow further. Though stopping short of formally suspending his campaign, retired neurosurgeon Ben Carson told supporters Wednesday that he does not see a “path forward.”

The non-Trump candidates hope to prevent him from acquiring the 1,237 delegates needed to secure the nomination. That would push an ultimate decision to the Republican National Convention in July, potentially turning the Cleveland showcase into a hothouse of intrigue and chaos.

The emergence of Romney as a leading Trump antagonist stoked speculation that he might offer himself as a consensus candidate at the convention. But loyalists were adamant that he has no plans to run.

Romney’s associates said he is not planning to offer an endorsement when he speaks Thursday morning at the University of Utah in Salt Lake City.

Even with fewer people in the race, some Republicans declared that Trump was nearly unstoppable.

Mike Murphy, who ran a pro-Jeb Bush super PAC, said the Trump “train may have left the station. I don’t want to be a critic of what’s being tried, but after millions of dollars in ads, it’s more important to narrow the field than to air more ads against him.”

Still, other operatives worked behind the scenes Wednesday on a ruthless ad blitz to discredit Trump by attacking his business career and character. It marks a dramatic escalation of an anti-Trump campaign that until recently had little firepower.

The air assault is largely being funded by Conservative Solutions PAC, a super PAC allied with Rubio; Our Principles PAC, a new anti-Trump outfit; American Future Fund, an Iowa-based nonprofit; and the super PAC arm of the conservative Club for Growth.

“We have a very target-rich environment,” said Katie Packer, who runs Our Principles. “He has left quite a wake of victims in his path.”

Packer said the increased focus on Trump is taking a toll on the front-runner, pointing to Texas Sen. Ted Cruz’s victories Tuesday in Texas, Oklahoma and Alaska.

People with knowledge of the group’s activities said a substantial number of new donors have come aboard since January, when Our Principles launched with an initial $3 million donation from Marlene Ricketts, the matriarch of the family that owns the Chicago Cubs. Among the new contributors are billionaire investor Paul Singer, who serves as the Rubio campaign’s national finance chairman.

New contributions are also bolstering the efforts of American Future Fund, a politically active nonprofit that launched a trio of anti-Trump ads online last week.

As of Tuesday, super PACs and other independent groups had plowed $16 million into commercials and mailers explicitly going after Trump and an additional $9.4 million into ads that refer to the brash billionaire.

Some of the operatives have been poring over polling data showing that only a small portion of the electorate was aware of negative aspects of Trump’s career.

“Small percentages of those surveyed knew about Trump University, the failure of Trump Mortgage, the KKK controversy,” said Rick Hohlt, a longtime Republican donor.

That has convinced some activists that they can gain traction against Trump. But even some involved in the new efforts are uncertain whether the coming assault will have an impact.

“You’re going to see a massive ad spend to the tune of tens of millions of dollars,” said one Republican involved in the planning, who spoke on the condition of anonymity to speak candidly. “Fundraising is not a problem. There may be other problems. Trump may be Teflon – this may not stick to him. On March 15, we will have an experiment: What happens when millions and millions of dollars go after Trump?”

Some party strategists said none of the attempts to trip up Trump will work unless Rubio, Cruz and Ohio Gov. John Kasich step up their campaigns.

“People sitting around here talking, it’s just a parlor game,” said Charlie Black, a longtime Republican strategist. “There’s nothing you can do behind the scenes. It’s all got to happen out there on the playing field. You’ve got to go beat the guy.”

What Rubio, Cruz and Kasich now are counting on most is depriving Trump enough delegates to force a convention showdown. The prospect of a brokered convention probably overstates what would unfold, in part because there is no sign of the ability of a few powerbrokers to have their way.

“This is a political marketplace with a set of structured rules,” Leavitt said. “Whoever can get 1,237 delegates will be the nominee. There is a lot of maneuvering within those rules that can occur. But there is no smoke-filled room.”

The convention rules will not be finalized until just before the event opens in Cleveland. About two-thirds or more of the delegates will be bound on the first ballot to back the candidate who won in their state or district. After that, however, they become free agents.

But the possibility of the convention delegates going against the candidate who had amassed the most delegates, even if not a majority, could leave the party even more divided and demoralized heading to the general election.

The convention rules may be moot if the tide keeps lifting Trump. New examples emerged Wednesday of party elites gravitating toward the former reality television star.

Stephen Moore, a conservative economist and former member of the Wall Street Journal editorial board, said he is considering an endorsement. “For me, Trump potentially represents a big expansion of the Republican Party, a way to bring in those blue-collar Reagan Democrats,” Moore said. “That’s necessary if the party is going to win again.”