Berkley is likely to face a tough general election opponent in Sen. Dean Heller, R-Nev., and. Part of Berkley's challenge is to be able to attract moderate voters outside of her more liberal Las Vegas-based district, and not backing a clean increase is a step in that direction.
Donnelly faces an uphill climb in a state where Republicans have enjoyed recent success at the statewide level. His best hope for victory in the GOP-leaning state is that Treasurer Richard Mourdock knocks off Sen. Richard Lugar, R-Ind., in the GOP primary. Donnelly needs to come across as a conservative-minded Democrat, and this vote is a step in that direction.
Rep. Martin Heinrich was the exception to the rule -- he voted for the measure despite running in a battleground state. But he also faces a competitive primary from state Auditor Hector Balderas.
Democratic Senate candidates running in more-liberal states -- Reps. Chris Murphy, D-Conn and Mazie Hirono, D, Hawaii - voted for the measure. Hirono and Murphy are both running to win over the Democratic base in their respective primary opponents. Murphy faces former Secretary of State Susan Bysiewicz and Hirono is facing former Rep. Ed Case, D-Hawaii.