I'll just let Klein explain it, because I agree:
Rep. Bart Stupak has not had an easy year. He made himself a villain to liberals when he threatened to kill the Affordable Care Act unless the already-restrictive abortion language was made more restrictive. He made himself a villain to conservatives when he accepted an executive order saying that the abortion language in the bill was intended to be extremely restrictive. And now, after weeks of negative ads and angry letters and shouting phone calls, he's retiring.
But Stupak isn't alone. If Sen. Ben Nelson was up for reelection this year, there's a good chance he'd be retiring too. The two of them took the most damage during health-care reform, and for the same reason: They took a hostage and then accepted the ransom. And while that strategy might have worked in the past, it's proven a disaster.
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Compare Nelson and Stupak to people such as Mark Warner or Brad Ellsworth, both of whom are moderate Democrats who had serious concerns about the bill, but who spent their time quietly getting those concerns addressed rather than using them to get TV bookings in advance of a high-profile deal. Nelson and Stupak made themselves into targets for both the left and the right, and ended the process with lots of notoriety but even more new enemies. Warner and Ellsworth haven't suffered from the same backlash. The old model in which moderate Democrats justify their vote for a bill by talking trash about it until they get bought off doesn't work in an environment where the media and the political opposition is waiting to pounce on the buy-off.
http://voices.washingtonpost.com/ezra-klein/2010/04/health-care_reforms_biggest_lo.html