How do we know that it’s the health-reform bill that’s to blame for the low poll numbers for Democratic Senate candidates and not just that these are more conservative states?

First, we asked voters how their incumbent senator voted on the health-care bill that passed on Christmas Eve. About two-thirds answered correctly. Even now, long before Senate campaigns have intensified, voters know where the candidates stand on health care. And second, we asked voters about their preference for Democrat versus Republican candidates in a generic House race. As in the Senate, the higher the level of opposition to health reform, the greater the likelihood that the state’s voters supported Republicans.