From Nate Silver in FiveThirtyEight:

Unions, activist groups and Democratic thought leaders are all starting to urge primary challenges against Democrats who vote against the health care bill. In many cases, it's an idle threat. Want to primary Dennis Kucinich, for instance? It's too late: Ohio's filing deadline passed in February.

I identified 10 Democrats, however, for whom all of the following criteria are true:

1. In a state in which the filing deadline for primary candidates is April 1st or later. I assume that at least some lead time is required to launch a primary challenge. This eliminates a number of important states, including California, Illinois, Ohio, and Pennsylvania.
2. Not yet committed yes votes on health care. According to David Dayen's whip count.
3. In districts with a PVI of R+4 or bluer. This is a critical threshold because Nancy Pelosi could permit all non-retiring Democratic members in districts with PVI's of R+5 or redder to vote against the bill and still have (barely) enough votes for passage. Everyone in a district of R+4 or bluer would have to vote for the bill, however.
4. Been in office for less than 10 years. It's very hard to primary incumbents who have a decade or more of service time in their districts, except in the case of gross misconduct, etc.
5. Are not retiring.

Here is that list:

[Number seven Silver lists as Scott Murphy, the Democratic Congressman who represents Greene and Columbia counties as well as the rest of NY-20. Murphy voted against the first version of the health care bill that passed the U.S. House in December. Silver reports that Murphy is leaning towards voting yes on the new health care bill, and only has $700,000 in the bank to ward off a possible Democratic primary challenger if he votes against the bill, and the same amount to fight one of four Republicans who are vying to get the chance to roll with a possible large Republican tide in the fall.]

Six of the ten vulnerable Democrats are in New York State, which offers a number of advantages to primary challengers: it has a very late filing deadline, and it has a lot of mobilizing infrastructure between strong unionization, the presence of the Working Families Party [The Working Families party says it will decline their line to anyone who votes against health care. Murphy was presented on Sunday with 7,000 signatures gathered by the WFP urging him to support the bill.], and the number of activist groups working in and around New York City. It's hard to imagine that the threat of a primary challenge wouldn't be at least a little bit persuasive to NY-24's Mike Arcuri, for instance, who has just $400,000 in cash on hand and voted for the health care bill originally. The unions have also explicitly threatened a primary challenge against NY-13's Mike McMahon, although he hasn't moved off his no vote so far....

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