That’s what Hadley Arkes argues:

As Stupak estimates the Democrats standing with him, he reportedly thinks he can count on 10-12 votes to resist a Senate bill that rejects his amendment and supports abortion. That could be enough to defeat a bill that passed in the House by only five votes. But another savvy watcher of Capitol Hill tells me that Stupak may have only four or five votes. The pro-life votes among the Republicans will hold. Not so for the pro-life Democrats: they will be subject to all of the pressures that emanate from a party fiercely pro-abortion – and in power. Benefits will be dangled to buy them off, and threats may be made to deprive them forevermore of projects and funds for their districts. And so, the pro-life Democrats are melting away. But their demise was foretold by the logic they accepted in remaining in the party. They had to absorb the notion that the rightness of abortion could not be challenged in principle, that the protection of nascent life would always be a peripheral concern, always giving way to something more pressing. And the pro-lifers who keep voting to sustain them in that position have had to talk themselves into the same moral confusion.