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

Democrats saw the sweeping health 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 at his weekly media briefing. "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."

The areas of disagreement have been more obvious. Senate Republicans on Tuesday rejected the White House plea for a simple up-or-down vote on Obama's health care plan, indicating they would offer hundreds of amendments to stop the legislation.

"Our constituents don't want the Senate to just wave the same thing through just because it has a new name and even more spending," said Don Stewart, a spokesman for Republican Leader Mitch McConnell of Kentucky.

Insurance market reforms like 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 a requiring nearly everyone to be insured. Smaller measures could be done individually, such as money for insurance pools to provide coverage to people with health problems.

Two days before Obama's televised health summit with Republicans and Democrats, the prospects for any bipartisan deal dimmed as the administration set the stage for pushing ahead with only Democratic support, a risky move that would require the president's political capital and elusive unity from a fractious party.

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 an inefficient health care system. Majority Democrats were on the verge of meeting the challenge before Republican Scott Brown's upset win in a Massachusetts Senate seat last month.

Brown's win underscored the perilous political environment for Democrats in an election year, but Obama didn't scale back his ambitions, opting for one last attempt at full-scale legislation. It costs around $1 trillion over a decade, requires nearly everyone to be insured or pay a fine, and puts new requirements on insurance companies, including — in a new twist responding to recent rate hikes — giving the federal government authority to block big premium increases.

If Obama fails on a comprehensive health care overhaul where Bill Clinton and other presidents failed before him, the chance won't come around again anytime soon.

The whole endeavor will now rise or fall on Obama's ability to sell his plan at the summit Thursday, and the reaction from lawmakers and the public in the days ahead.

Some rank-and-file Democrats were openly skeptical that the White House and congressional leaders could pull it off. Rep. Jason Altmire, D-Pa., a moderate who opposed the health legislation when it passed the House, questioned whether Speaker Nancy Pelosi could hang on to the votes that allowed her to get the bill through 220-215 in November. Since then a couple of Democrats have left the House, and Pelosi may also lose votes from anti-abortion Democrats who oppose the less restrictive abortion language in the Senate bill, which Obama kept in his plan.

"Is she going to be able to hold everybody that was for it before?" Altmire asked. "What about the marginal members in the middle who got hammered over this vote and would love a second chance to perhaps go against it?"

Only 32 percent of Americans say Congress should move soon to pass a comprehensive bill, embodied in the House and Senate Democratic legislation and Obama's new plan. That was the finding of a poll released Tuesday by the nonpartisan Kaiser Family Foundation. Americans were evenly divided on the legislation, in a 43-43 percent split.

Most still want Congress to pass something this year, and 58 percent say they'll be disappointed or angry if that doesn't happen. But 20 percent say lawmakers should pass a scaled-back bill, and 22 percent say it would be a good idea to call a time-out on health care and come back later in the year.

Obama's plan does not include the government insurance option sought by liberals and it dramatically scales back a tax on high-value insurance plans from the Senate bill that was opposed in the House. It eliminates a controversial Medicaid deal for Nebraska, offers all states more help with Medicaid funding, and beefs up subsidies to help lower-income people buy care, all changes that won praise from House Democrats. It also closes the so-called "doughnut hole" in Medicare's prescription drug coverage.

Individuals and small businesses would shop for insurance in regulated state-based marketplaces called exchanges.