Sens. Jon Tester (Mont.) and Claire McCaskill (Mo.), the two most vulnerable Democratic senators, have declined to endorse Obama’s call for the legalization of gay marriage.

Sens. Joe Manchin (W.Va.), Bob Casey (Pa.) and Bill Nelson (Fla.), Democrats who have easier races but in states that could become more competitive by November, have also backed away from Obama’s stance.

They all represent states with constitutional amendments or laws banning same-sex marriage…

The exception is Rep. Sherrod Brown (D) who faces a stiff challenge in Ohio, where outside groups have spent more than $5 million on ads to defeat him.

Brown issued a strong statement siding with Obama, even though his home state has a constitutional amendment banning same-sex marriage.