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 Hagan Former Sen. Kay Hagan in ICU after being rushed to hospital GOP senator floats retiring over gridlock 2016’s battle for the Senate: A shifting map 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 FrankenFranken emerges as liberal force in hearings Trump nominees dodge 'climate denier' charge Justice requires higher standard than Sessions 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 KirkGOP senator: Don't link Planned Parenthood to ObamaCare repeal Republicans add three to Banking Committee Juan Williams: McConnell won big by blocking Obama MORE (R-Ill.), Kelly AyotteKelly AyotteTen rumored Trump Cabinet picks who didn't get a job Sasse, Perdue join Armed Services Committee Avid pilot among GOP senators joining Transportation committee MORE (R-N.H.) and Dean HellerDean HellerMnuchin weathers stormy confirmation hearing Live coverage: Senators grill Trump's Treasury pick Five things to watch for in Mnuchin hearing 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: Tech listens for clues at Sessions hearing | EU weighs expanding privacy rule | Senators blast Backpage execs Senate rejects Paul's balanced budget Republicans add three to Banking Committee MORE (R-Kans.), as well as both of his NRSC deputies, Sens. Rob PortmanRob PortmanHillary gives Bernie cool reception at Trump inaugural lunch GOP governors defend Medicaid expansion Senators introduce dueling miners bills MORE (R-Ohio) and Ted CruzTed CruzTrump's America: Businessmen in, bureaucrats out When Trump says 'Make America Great Again,' he means it Booker is taking orders from corporate pharmaceuticals 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.