Four Democratic senators facing tough reelection fights voted in favor of the immigration reform measure on Thursday afternoon, opening themselves up to likely attacks from conservatives heading into 2014 and beyond.
Sen. Al Franken (D-Minn.), who is also up for reelection but considered less vulnerable, also voted for the bill. No Democrats voted against the legislation.
A handful of Republicans in blue and swing states — some of who could see primary challenges — voted for passage.
Sen. Lindsey Graham (S.C.), up for reelection in 2014, voted for the bill.
He has long faced criticism from conservatives for his record, though no primary challenger has yet emerged, and he's long been an outspoken supporter of reform.
Sens. Kelly Ayotte (N.H.), Marco Rubio (Fla.) aren't up for reelection until 2016, but their support for the reform measure is already causing consternation among conservatives.
Most notably, Tea Party leader Sarah Palin suggested the two should face primary challenges.
And Rubio, who is seen as a probable 2016 contender, has seen his poll numbers drop in the past month, as debate over the bill intensified.
Critics say his role in crafting and selling the measure to the public could hurt him with Republican primary voters and jeopardize his presidential aspirations.
Also up for reelection in 2016 is Sen. Mark Kirk (R-Ill.), whose vote for the bill could help him appeal to centrist voters in his blue state.
Sen. Dean Heller (R-Nev.), up in 2018 in a swing state, will also need to appeal to that voting bloc.
GOP debate over the bill has highlighted a split within the party on reform. Conservatives generally oppose what they say is a temporary stop-gap measure that does little to solve the problem and offers inadequate border security provisions.
The no-votes from National Republican Senatorial Committee Chairman Jerry Moran (Kans.) and his NRSC deputies, Sens. Rob Portman (Ohio) and Ted Cruz (Texas), underscore what a politically perilous issue immigration may be for some Republicans going forward.
In the House, a large contingent of far-right members have pledged to oppose the Senate measure.