President Obama and some Democratic congressional leaders had pledged to involve Republicans in health care reform negotiations, but it is looking increasingly likely that bipartisanship will be among the casualties of the rush to approve a bill.

Obama told Congress on Friday not to "lose heart" in moving quickly to hammer out legislation that would check rising health care costs and cover millions of uninsured Americans without adding to the federal deficit.

But Republican proposals have gone nowhere in Congress, and the GOP isn't signing on to the Democrats' proposals -- and that didn't stop Obama from heralding "unprecedented progress." 

Three of the five congressional committees working on health care legislation passed their versions of the Democratic plan this week without winning over a single Republican vote. The House Energy and Commerce Committee and the Senate Finance Committee are still discussing the proposals.

Democrats facing tough re-election bids or representing conservative districts are demanding additional measures to hold down costs. They have been unnerved by the nonpartisan Congressional Budget Office declaring that the legislation taking shape so far would not prevent federal spending on health care from rising.

Republicans have seized on those remarks as ammunition.

"When are Democrats going to admit that their claims about their government-run plan are pure fiction? Repeating the same disproven myths over and over again will not make them true," said Michael Steel, spokesman for House Minority Leader John Boehner of Ohio.

"Instead, Democrats should scrap their costly, job-destroying proposal and work with Republicans on a real plan to give Americans better access to affordable health care."

Yet Obama is soldiering on in his quest to get members of both political parties on board with his top domestic priority.

So far, he has summoned Republicans and Democrats to the White House. He has used public forums to make a direct pitch to the people. He has turned to his political operation to air campaign-like TV ads.

But it will take a lot to convince Republicans, nearly united in opposing the Democrats' plan.

"It would empower Washington -- not doctors and patients -- to make health care decisions and would impose a new tax on working families during a recession," Arizona Sen. Jon Kyl said in the GOP's weekly address. "They propose to pay for this new Washington-run health care system by dramatically raising taxes on small business owners."

Kyl, the Senate's No. 2 Republican, said his party's proposed alternatives should be considered.

"These changes do not require government takeover of the health care system, or massive new spending, job-killing taxes or rationing of care," he said, seeking to string together the biggest fears of Obama's plan to challenge the popular president.

Obama rejected such criticism out of hand in his weekly Internet and radio address.

"Now, we know there are those who will oppose reform no matter what," Obama said. "We know the same special interests and their agents in Congress will make the same old arguments and use the same scare tactics that have stopped reform before because they profit from this relentless escalation in health care costs."

Obama must choose at some point whether to make concessions that could attract a few Senate Republican votes -- and anger liberal supporters. The alternative is a bare-knuckled parliamentary tactic that would inflame partisan tensions and probably kill some of the items he wants in the legislation.

On Wednesday he invited four Republican senators to the White House to discuss health care. Three -- Sens. Saxby Chambliss of Georgia, Bob Corker of Tennessee and Lisa Murkowski of Alaska -- are seen by colleagues as highly unlikely to vote for an Obama-backed plan.

The fourth, Sen. Susan Collins of Maine, is a moderate Republican viewed as a possible supporter, even though she has demanded changes in the Democratic-drafted bills.

Even those who accepted White House invitations said it's hard to imagine that Obama thinks such chats with conservatives will win him any votes.

"I think he's just trying to get a sense as to what the prognosis might be in the Senate," Murkowski said in an interview.

As for Obama's push to get the House and Senate to pass separate bills by August, she said, "I just don't see how it comes together."

Murkowski said the White House is sending a "mixed message" by coupling its Republican outreach with thinly veiled threats to use strong-arm tactics to ram home a health care bill if Republicans insist on too many changes.

Obama adviser David Axelrod is walking that line.

"We want to work with everyone who will work with us, and we want to do it in the spirit of bipartisanship," he said in an interview Thursday. But, he added, "We can't defer reform and we want to move forward. Those who don't, they need to address those Americans struggling with higher premiums and losing their insurance."

Senate Democrats could resort to a parliamentary procedure, known as "reconciliation," that essentially would bar Republicans from using stalling tactics to block a health care bill. But Senate rules would allow opponents to knock some nonbudgetary items from the bill. Those might include the "public option" for insurance, which is dear to many liberals.

"It's obviously better to have it bipartisan," said John Podesta, who headed Obama's transition team and advises on health care. "But there is a considerable amount that could be done, and will be done, with reconciliation" if Republicans don't come on board, he said.

The Associated Press contributed to this report.