Before that came the vote on a congressional deficit-cutting commission, which many GOPers had supported when it was a Republican idea...and suddenly opposed when it was a Democratic idea, including several who switched their votes for no reason other than that they were more politically vulnerable. Sen. Evan Bayh might have reconsidered his decision to retire if the Senate managed to pass the commission. He didn't, and suddenly, the country had a front-page example of a Democrat who could not bear both the liberals in his party and the obstructionist Republican colleagues.
Before that came the White House's decision to unleash the president's brain, and urge it be televised, as he did at the House Republican retreat in Maryland. It was a slam dunk performance, weapons of mass destructions were found, and the liberal base became ... enthusiastic. The White House push did not let up. Communications Director Dan Pfeiffer became a daily blogger, and although his posts were occasionally anodyne, they bear the imprint of an official White House declaration. Slowly but surely, the White House built a mechanism to shame Republicans.
Mixed into all of this (perhaps the largest single current) was the decision by some insurers to significantly raise their premiums on people with coverage -- that is -- on people who in theory are happy with the system and don't have any stake in reform. Obama has had trouble reaching these Americans, and the insurance industry gave him a gift. (Thanks, WellPoint!)
Incentivizing GOPers to cooperate on key votes was too hard a lift, but it provided cover for House Democrats worried about their reelection chances. The tone had truly shifted; Republicans had to play some defense, and it didn't look very pretty.
The Republican Party had a single strategy for the first half of this year: impede the progress of Democratic legislation. Their reasons were a mix of principle and politics, and they were unified in the cause. The Democrats and the White House seemed to have found a way to play on this overconfidence and call their bluff.
What Democrats now have as an incentive to vote against their short term political interests -- "the health care reform bill" is not popular, but reform is -- is that it has become a moral crusade of sorts. They've got to pass it to show the American people that they can govern. They've got to pass it to show the American people that Republicans have no good alternative to expanding coverage and cracking down on insurance companies. This is what is in their heads now -- it's why 10 Democrats (at least) who voted no in the House are willing to give an even more expensive bill (the Senate bill plus the Obama fixes) a second chance.