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.
The big question is whether the former senator has learned his lesson after refusing outside money in 2010, letting Johnson outspend him by a wide margin, and declining to air any attack ads. If Feingold is the nominee and runs a smarter campaign this time around, Johnson could face an uphill battle to hold his seat.
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