“I think we’re all vulnerable next year,” said Costa, who won with nearly three-quarters of the vote last year in a district that President Barack Obama carried with 60 percent.

Costa is one of a handful of moderate House Democrats from relatively stable districts who aren’t yet on board with the health care bill and whose “no” votes could force colleagues in more marginal districts to cast offsetting — and potentially perilous — “yes” votes.

Rep. Dennis Cardoza (D-Calif.) — from a 59 percent Obama district — is another. Rep. Henry Cuellar (D-Texas), who won with 69 percent of the vote in 2008 and has never gotten less than 59 percent, is also in play, calling himself “undecided.”

Brian Baird (D-Wash.), a politically safe liberal who was once a harsh critic of the legislation, said he has moved from leaning “no” to “undecided.”