During the era of GOP dominance, Democrats dreamed of forging a new majority and taking back the U.S. House of Representatives. They finally turned the dream into reality in the 2006 election, and broadened their majority in 2008. But as the old saying goes, "Be careful what you wish for."

Today they have a House majority at odds with itself. Case in point, health care reform. This signature issue is exposing fundamental rifts in the ranks. Conservative Democratic "blue dogs" are worried that the proposed overhail could be too expensive and government-intrusive, thereby ticking off conservative and moderate voters in swing House districts; moreover, the blue dogs' concerns are being echoed by the two dozen Democratic House freshmen who captured seats last November in districts that normally elect Republicans.

Then there are the House liberals, most of whom hail from safe liberal seats and  echo the sentiments of the party's liberal base. These folks are generally thought to be President Obama's most faithful followers. They have been pushing for sweeping health care reform - seeking a government-run insurance plan to compete with private insurers - for a very long time.

Well, we heard from the House liberals yesterday, big time. And they are seriously ticked off.

In a letter to House Speaker Pelosi, 57 liberal members assailed various watered-down provisions in the latest House plan, compromises that have been forged to please the blue dogs and moderate Democratic freshmen. Apparently the House deal would seriously weaken the proposed government health insurance plan (one New York liberal congressman says the public plan has been "eviscerated"), and the liberals see that as a sop to the private insurers.

The liberals are so angry that they're even threatening to vote No on any final House package that betrays what they call the party's "basic values." If those 57 liberals follow through on their threat, their numbers are sufficient to kill a House bill and therefore scuttle health care reform this year.

Pelosi yesterday tried to put a happy face on the intramural tensions: "We have tremendous diversity, whether it's generational, geographic, philsophical, ethnic, gender, you name it. It is a great kaleidoscope." But Democratic infighting has helped kill health care reform in the past - as far back as the early 1970s - and Pelosi can hardly be pleased that these 57 members of the Progressive Congressional Caucus consider the latest compromises to be "fundamentally unacceptable...a large step backwards." The letter to Pelosi concludes: "We simply cannot vote for such a proposal."

Then the liberals held a press conference. Congresswoman Lynn Woolsey of California, who co-chairs the Progressive Caucus, said: "Many of us favor a single-payer system (by which the government would control the delivery of health care). We have compromised. We can compromise no more." Her co-chair, congressman Raul Grijalva of Arizona, said: "I think that, after today, (President Obama and Democratic congressional leaders) are going to have to accept our seriousness and deal with us whether they like it or not."

Some of this is the usual Capitol Hill bluster, the kind we would expect to hear in any high-stakes negotiation. Early today, in fact, the bluster seemed to have helped the liberals; the latest-latest-latest version of the House deal makes it a bit easier for low-income Americans to buy health insurance with the help of government subsidies (the pact with the blue dogs had minimized those subsidies, much to the ire of the liberals). Indeed, it's hard to imagine that the House liberals would ultimately follow through on their Thursday threat and vote No on the House floor, thereby killing off the Obama administration's signature issue. The White House will likely tell them that half a loaf (which is likely to be negotiated with the Senate, in the end) is surely better than nothing, that an imperfect health care deal would at least give them something to build on in the future - and that, politically speaking, their own president would be seriously damaged if his own liberal allies stood against him. Given such a scenario, would the House liberals really vote No on the floor? The White House could well call their bluff.

Such are the current pitfalls of Pelosi's "great kaleidoscope" - or, as the dictionary defines it, "a continually changing pattern of shapes and colors." The big question is whether the diverse Democratic majority can halt the oscillation and cohere on health care.