Updated

Party leaders are quickly finding out that you can't please all Democrats all the time, as they try to reach an accord on health care reform.

One day after House Democratic leaders struck what was seen as a breakthrough deal with a handful of key moderates, the liberal wing of the party lined up Thursday against the compromise.

Fifty-three lawmakers from the Congressional Black Caucus, the Progressive Black Caucus and the Congressional Hispanic Caucus have sent a letter to House Speaker Nancy Pelosi and key committee chairmen formally opposing the deal that House leaders struck with a group of fiscally conservative Democrats known as Blue Dogs.

The letter, written to express their "opposition" to the negotiated deal, called the agreement "fundamentally unacceptable" and a "large step backwards."

"We're going to fight this with every effort that we have," CBC Chairwoman Barbara Lee, D-Calif., told reporters Thursday.

"We're not obstructionists. ... We're not here to embarrass or divide our party," said Rep. Raul Grijalva, D-Ariz.

But he and other liberal Democrats, who have voiced objections to the deal since it was announced Wednesday, said that without a meaningful public plan included in the final bill, they cannot support it.

The threats and the letter Thursday only amplified their criticism and were an indication that the deal with the Blue Dogs could cause more problems than it solves.

Under the agreement, the bill would steer away from using Medicare as the blueprint for a proposed government insurance option. It would reduce federal subsidies to help lower-income families afford coverage and would exempt additional businesses from a requirement to offer health insurance to their workers.

Rep. Mike Ross of Arkansas, a leader of the conservative Democrats, said the changes agreed to by the leadership in the House bill would cut its cost by about $100 billion over 10 years.

The agreement, though, was hardly a broad-based deal. It was only struck between party leaders and four Blue Dog Democrats, who sit on the critical House Energy and Commerce Committee.

That committee is the only House panel that has not passed a version of health care reform legislation. Though a total of seven Blue Dogs sit on that committee, the four who signed on to the deal would give the committee's chairman, Rep. Henry Waxman, D-Calif., enough votes to pass the bill out of committee if the rest of the party stays in line.

But the fractures continue. Many in the Blue Dog Coalition, which has 55 members in the House, still have concerns over the deal, saying it doesn't go far enough to cut costs. Republicans certainly aren't appeased. And now liberals are peeling off.

After Wednesday's announcement, the legislation advanced slowly Thursday in the Energy and Commerce Committee.

The panel plowed methodically through a stack of proposed changes to the bill. Waxman, presiding over the session, warned lawmakers against offering amendments that make the bill more expensive, and he agreed to a Republican suggestion to limit the time allowed for debate. He said he hopes to finish the bill sometime Friday, and House leaders have promised to bring it up for a vote in the full House in September, after the congressional August break.

Pelosi expressed confidence the committee would approve the bill, and she said the full House would follow suit in the fall. She also signaled flexibility on key issues, saying that despite her own backing for abortion rights, she would not allow the issue to torpedo legislation.

But Waxman's shaky majority was on display early, when the committee voted 29-28 to defeat a Republican amendment to strengthen ID requirements designed to prevent illegal immigrants from getting Medicaid benefits.

Another controversial Republican amendment passed by voice vote, over Waxman's objections. Backed by Rep. Mike Rogers, R-Mich., it would bar the federal government from using research comparing medical treatments' effectiveness to deny or ration care.

Already, Congress is running behind a timetable set by its leaders and the White House for a health care bill, and one veteran senator warned of additional slippage.

"The president wanted to have it on his desk in October," said Sen. Tom Harkin, D-Iowa. "He'll probably have it in November now. But I'm very hopeful we'll get it done at least by that time."

Senior aides and lawmakers said privately they thought Harkin was being overly optimistic, and they warned of work spreading late into December on the highly controversial issue.

On the Senate side, three Republicans are still negotiating with Democratic members of the Finance Committee toward a bipartisan bill.

Sen. Mike Enzi, R-Wyo., one of six lawmakers involved in bipartisan talks, said the legislation wasn't "ready for prime time."

Meanwhile, the Senate's top Republican accused Democrats of cutting Medicare to finance a "massive new government-run plan."

"Some in Congress seem to be in such a rush to pass just any reform, rather than the right reform, that they're looking everywhere for the money to pay for it, even if it means sticking it to seniors with cuts to Medicare," said Sen. Mitch McConnell, R-Ky., in the latest in a series of daily Senate speeches on health care.

FOX News' Chad Pergram and The Associated Press contributed to this report.