Pro-Choice, Pro-Stupak?

Nate Silver studies the Stupak amendment vote:

I was surprised at the number of Democrats who have solid pro-choice voting records but who nevertheless voted for Stupak Amendment. And the vast majority of these Democrats voted for, not against, passage of the underlying health care bill...

I'm sure there are idiosyncratic explanations in a number of cases, but I take this as a sign that they're worried about the re-election environment they'll face in 2010. 11 of the 20 pro-choice Democrats who voted for Stupak reside in districts that are rated as vulnerable according to Cook Political (note: candidates who are leaving the House to run for Senate or governor are rated based on those races instead). And, interestingly, they seem to think that a pro-choice vote would render them more vulnerable than a pro-health care vote, even though the pro-choice position is generally more popular than the health care bill on the table at the moment (although some recent polls have shown the pro-choice position losing ground).