1. Sen. Mark Kirk (R-Ill.)
Kirk narrowly defeated a scandal-plagued Democratic candidate by less than 2 points in the 2010 Republican wave election. In the 2016 presidential election year, he will likely face a much more Democratic electorate.
The senator has worked hard to bolster his bipartisan credentials, backing immigration reform, gun control and environmental protections. He was also the second Republican senator to embrace gay marriage.
His hard-fought battle to return from a debilitating stroke could also earn him sympathy from voters….
2. Sen. Ron Johnson (R-Wis.)
Polls show Johnson isn’t that well-known or well-liked in Democratic-leaning Wisconsin, where he won by 5 points in the 2010 GOP wave.
He may face a rematch against Sen. Russ Feingold (D-Wis.) in a high-turnout election year. Adding to his worries: No GOP presidential nominee has carried Wisconsin in more than three decades.
Johnson has done little to build bipartisan credentials in the Democratic-leaning state, and Feingold is giving serious consideration to another run.