Sen. Mark PryorMark PryorCotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm Top Democrats are no advocates for DC statehood MORE (Ark.) and other vulnerable Senate Democrats up for reelection this year voted yes in a key test vote Monday on immigration reform.

Every Democrat present voted to advance a border security measure that was added to the underlying Senate bill.

Beesides Pryor, Sens. Mark BegichMark BegichThe future of the Arctic 2016’s battle for the Senate: A shifting map Trump campaign left out of Alaska voter guide MORE (D-Alaska), Kay HaganKay HaganGOP senator floats retiring over gridlock 2016’s battle for the Senate: A shifting map The untold stories of the 2016 battle for the Senate MORE (D-N.C.) and Mary LandrieuMary LandrieuFive unanswered questions after Trump's upset victory Pavlich: O’Keefe a true journalist Trump’s implosion could cost GOP in Louisiana Senate race MORE (D-La.) all supported the bill. Sen. Al FrankenAl FrankenDems press Trump to keep Obama overtime rule GOP wants to move fast on Sessions Overnight Cybersecurity: Lawmakers pushing for vote to delay warrant rule changes MORE (D-Minn.), who is seen as less vulnerable, also voted yes.

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So did Sen. Brian Schatz (D-Hawaii), who's likely to face a tough primary against Rep. Colleen Hanabusa (D-Hawaii).

A number of swing- and blue-state Republicans who could face tough reelection battles in future years joined them

Sens. Mark KirkMark KirkBattle for the Senate: Top of ticket dominates The untold stories of the 2016 battle for the Senate Women make little gains in new Congress MORE (R-Ill.), Kelly AyotteKelly AyotteBattle brews over Trump’s foreign policy Battle for the Senate: Top of ticket dominates NH voters hold Ayotte accountable for gun control votes MORE (R-N.H.) and Dean HellerDean HellerGovernments and businesses: Teaming up for taxpayers GOP senator won't rule out 2018 run for Nevada governor A holiday surprise: Will Congress protect privacy? MORE (R-Nev.) were among the 15 Republican yes votes. Kirk and Ayotte are up for reelection in 2016, while Heller won't face the voters until 2018.

Some notable opponents: National Republican Senatorial Committee Chairman Jerry MoranJerry MoranOvernight Tech: Bill protecting online reviews heads to Obama | New addition to FCC transition team | Record Cyber Monday Overnight Finance: Trump expected to pick Steven Mnuchin for Treasury | Budget chair up for grabs | Trump team gets deal on Carrier jobs Congress passes bill protecting online customer reviews MORE (R-Kans.), as well as both of his NRSC deputies, Sens. Rob PortmanRob PortmanGOP debates going big on tax reform Who is Tim Ryan? A closer look at Pelosi’s challenger Battle for the Senate: Top of ticket dominates MORE (R-Ohio) and Ted CruzTed CruzCruz: I'd rather have Trump talk to Taiwan than Cuba or Iran Lewandowski: Top Cruz aide advised Trump team before NH primary Five reasons why Donald Trump could be the 'Greatest Communicator' MORE (R-Texas).

Cruz has been a strident opponent of the bill, but Portman was viewed as a gettable vote for the amendment's advocates. Sen. Pat Toomey (R-Pa.), who's likely to face a tough race in 2016, also voted against the amendment.

It’s unclear whether some of those who voted yes on Monday will vote no in a final vote later this week.

It’s also possible that some Republicans who voted no on Monday could reverse themselves later in the week. Voting no on Monday could allow some senators to retain leverage. Portman, for example, is pushing for language on immigration enforcement to be added to the bill.