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.

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Sen. Mark PryorMark PryorMedicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm MORE (D-Ark.), a top GOP target in 2014, voted to advance the bill along with vulnerable Sens. Mark BegichMark BegichPerez creates advisory team for DNC transition The future of the Arctic 2016’s battle for the Senate: A shifting map MORE (D-Alaska), Kay HaganKay HaganLinking repatriation to job creation Former Sen. Kay Hagan in ICU after being rushed to hospital GOP senator floats retiring over gridlock MORE (D-N.C.) and Mary LandrieuMary LandrieuMedicaid rollback looms for GOP senators in 2020 Five unanswered questions after Trump's upset victory Pavlich: O’Keefe a true journalist MORE (D-La.)  

Sen. Al FrankenAl FrankenWhat killing net neutrality means for the internet Overnight Tech: Net neutrality fight descends into trench warfare | Zuckerberg visits Ford factory | Verizon shines light on cyber espionage Franken, top Dems blast FCC over net neutrality proposal MORE (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 GrahamLindsey GrahamTop admiral: North Korea crisis is 'worst I've seen' Comey to testify before Senate Judiciary Committee Overnight Defense: US moving missile defense system to South Korea | Dems want justification for Syria strike | Army pick pushes back against critics of LGBT record MORE (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 AyotteKelly AyotteBottom Line How Gorsuch's confirmation shapes the next Supreme Court battle THE MEMO: Trump set to notch needed win with Gorsuch MORE (N.H.), Marco RubioMarco RubioOvernight Defense: Commander calls North Korea crisis 'worst' he's seen | Trump signs VA order | Dems push Trump to fill national security posts What’s with Trump’s spelling mistakes? Boeing must be stopped from doing business with Iran MORE (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 KirkMark KirkThe way forward on the Iran nuclear deal under President Trump ObamaCare repeal bill would defund Planned Parenthood Leaked ObamaCare bill would defund Planned Parenthood MORE (R-Ill.), whose vote for the bill could help him appeal to centrist voters in his blue state. 

Sen. Dean HellerDean HellerOvernight Energy: Trump orders review of national monuments, claiming ‘egregious abuse’ Draft House bill ignites new Yucca Mountain fight Ex-Nevada state treasurer may challenge Heller in 2018 MORE (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 MoranJerry MoranIT modernization bill reintroduced in Congress At the table: The importance of advocating for ABLE GOP lawmakers lead way in holding town halls MORE (Kans.) and his NRSC deputies, Sens. Rob PortmanRob PortmanTrump talks big on trade, but workers need action Trump tax plan prompts GOP fears about deficit Overnight Regulation: Senators call for 'cost-effective' regs | FCC chief unveils plans to roll back net neutrality MORE (Ohio) and Ted CruzTed CruzTrump in campaign mode at NRA convention Trump’s hands are tied on 9th Circuit Schumer: Trump's handling of North Korea 'all wrong' MORE (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.