WASHINGTON – Democratic congressional leaders confronted the reality Tuesday that they may not be able to pass the comprehensive health care overhaul sought by President Obama. Republican leaders prepared to do everything in their power to make sure they can’t.

Democrats saw the sweeping bill that Obama unveiled ahead of a bipartisan health care summit Thursday as their last, best chance at a top-to-bottom remake of the nation’s health care system that would usher in near-universal health coverage.

But some were clear-eyed about the difficulties after a year of corrosive debate and the loss of their filibuster-proof supermajority in the Senate.

House Majority Leader Steny Hoyer, D-Md., said comprehensive reform would be best, but it’s not all or nothing.

”We may not be able to do all. I hope we can do all, a comprehensive piece of legislation that will provide affordable, accessible, quality health care to all Americans,” Hoyer said. ”But having said that, if we can’t, then you know me — if you can’t do a whole, doing part is also good. I mean, there are a number of things I think we can agree on.”

House Speaker Nancy Pelosi was a bit more optimistic about the prospects for Obama’s plan.

”I think it is getting a good reception in our caucus, but nonetheless we have more work to do to have everyone on board,” said Pelosi, D-Calif.

Republican leaders said they would attend the summit but see no point in the session, arguing that Obama and the Democrats are determined to ram their comprehensive bill through Congress using special parliamentary rules.

”We’re happy to be there, but I’m not quite sure what the purpose is,” said Senate Republican Leader Mitch McConnell of Kentucky, who invited some of Obama’s fiercest critics to join him Thursday, including Sens. John McCain, R-Ariz., and Tom Coburn, R-Okla.

Senate Republicans also rejected the White House plea for an up-or-down vote on Obama’s health care plan, indicating they would offer hundreds of amendments to stop the legislation.

Insurance market reforms such as barring insurers from denying coverage to people with pre-existing conditions would be difficult or impossible to pull off without a large risk pool achieved by requiring nearly everyone to be insured. Smaller measures could be done individually, such as money for insurance pools to provide coverage for people with health problems.

Obama’s new plan used legislation already passed by the Senate as its starting point, making changes designed to appeal to House Democrats. He unveiled it Monday, almost exactly a year after calling on Congress to act to reform the nation’s costly and inefficient health care system.