Sen. Mark PryorMark Lunsford PryorMedicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm 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 Peter 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 Ruthven HaganPolitics is purple in North Carolina Democrats can win North Carolina just like Jimmy Carter did in 1976 North Carolina will be a big battleground state in 2020 MORE (D-N.C.) and Mary LandrieuMary Loretta LandrieuProject Veritas at risk of losing fundraising license in New York, AG warns You want to recall John McCain? Good luck, it will be impossible CNN producer on new O'Keefe video: Voters are 'stupid,' Trump is 'crazy' MORE (D-La.) all supported the bill. Sen. Al FrankenAlan (Al) Stuart FrankenShould the Rob Porter outcome set the standard? Grassley, Dems step up battle over judicial nominees Sen. Gillibrand, eyeing 2020 bid, rankles some Democrats 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 Steven KirkHigh stakes as Trump heads to Hill Five things to watch for at Trump-Senate GOP meeting Giffords, Scalise highlight party differences on guns MORE (R-Ill.), Kelly AyotteKelly Ann AyotteAudit finds US Defense Department wasted hundreds of millions of taxpayer dollars US sends A-10 squadron to Afghanistan for first time in three years No, the US did not spend million on a gas station in Afghanistan MORE (R-N.H.) and Dean HellerDean Arthur HellerThe siren of Baton Rouge Big Republican missteps needed for Democrats to win in November What to watch for in the Senate immigration votes 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 MoranGerald (Jerry) MoranFlake to try to force vote on DACA stopgap plan Congress punts fight over Dreamers to March The 14 GOP senators who voted against Trump’s immigration framework MORE (R-Kans.), as well as both of his NRSC deputies, Sens. Rob PortmanRobert (Rob) Jones PortmanCommittee chairman aims for House vote on opioid bills by Memorial Day Flake to try to force vote on DACA stopgap plan Congress punts fight over Dreamers to March MORE (R-Ohio) and Ted CruzRafael (Ted) Edward CruzOvernight Health Care: Trump eases rules on insurance outside ObamaCare | HHS office on religious rights gets 300 complaints in a month | GOP chair eyes opioid bill vote by Memorial Day HHS official put on leave amid probe into social media posts Trump, Pence to address CPAC this week 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.