Updated 5:10 p.m. | Montana Sen. Jon Tester was the only red-state Democrat up for re-election this year to vote against a stopgap funding measure Monday to end the three-day government shutdown. 

And his vote, along with an earlier one to oppose advancing debate on the short-term continuing resolution, is already opening him up to Republican attacks that he sided with his party’s most liberal senators, including many 2020 hopefuls. 

But Tester is holding firm.

“It’s never been about DACA for me,” he said after the midday vote, referring to the Deferred Action for Childhood Arrivals program. “It’s been about community health centers, it’s about security on the southern border.”

Liberal groups have been pressuring lawmakers to oppose a CR that does not include protections for DACA recipients — immigrants brought illegally to the country as children, also known as Dreamers.

Tester is the only Democratic senator still in office who opposed the 2010 DREAM Act, which would have helped protect Dreamers from deportation. Latinos, who constitute the majority of DACA recipients, make up less than 4 percent of Montana’s population, according to the U.S. Census Bureau. 

Tester said Monday his concerns were about the unreliability of a short-term funding bill.

“We needed some commitments on funding for some pretty basic issues in rural America that were not done,” he said. 

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A former chairman of the Democratic Senatorial Campaign Committee, Tester is running for a third term in a state President Donald Trump won by more than 20 points in 2016. He also voted against confirming Neil Gorsuch  to the Supreme Court last year.

Tester has never earned more than 50 percent of the vote in his previous Senate races. He ranked No. 6 on Roll Call’s list of the most vulnerable Senate incumbents last November, a year out from the midterms. Inside Elections with Nathan L. Gonzales rates his re-election race Tilts Democratic.

The National Republican Senatorial Committee went after Tester’s image — he’s a working farmer in the state — to argue he’s no longer a good fit for Montana.

“Jon Tester’s insane shutdown antics have shown Montanans he’s not the same guy he used to be,” NRSC communications director Katie Martin said in a statement.

Watch: Working the Weekend — Highlights From the Shutdown Floor Debate

“Voting alongside the most extreme of the Democratic Party, it’s now crystal clear that Tester can no longer be trusted,” she added.

The Senate Leadership Fund, the super PAC aligned with Senate Majority Leader Mitch McConnell, pledged to hold Tester accountable. 

While several of the GOP’s top picks with statewide name recognition passed on challenging the Democrat, there’s still a crowded primary field.

State Auditor Matt Rosendale is seen as the NRSC’s preferred candidate. He also has the backing of Great America Alliance, which had ties to former Trump strategist Steve Bannon. Wealthy businessman and Air Force veteran Troy Downing has met with the White House and has been trying to claim the Trump family’s support. The primary is June 5.

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Over the weekend, Downing attacked Tester for putting “the status of illegal immigrants above our country’s national security.” Tester joined most Democrats and four Republicans on Friday night to block an earlier House-passed stopgap measure. 

Rosendale launched a similar attack Monday after the day’s first vote. 

“Jon Tester’s vote against re-opening the government makes it clear to the people of Montana that he cares much more about illegal immigrants than our military families at home or our children who rely on CHIP,” the state auditor said in a statement, referencing the Children’s Health Insurance Program.

Tester signed on to bipartisan legislation in the Senate last fall to reauthorize CHIP for five years. 

After the Friday vote, Rosendale blamed Tester for shutting down the government. The same press release tried to paint the senator as hypocritical, pointing to quotes from 2013 in which he criticized the idea of a government shutdown. 

When Rosendale launched his first bid for Congress in October 2013 — for Montana’s open House seat — local reporters asked him if he supported House Republicans using a partial government shutdown as leverage to push for defunding the 2010 health care law. 

“Drastic times call for drastic actions,” Rosendale told the (Helena) Independent Record at the time. “In order to find a solution, that’s why drastic measures are talking place — to force everyone to the table, to work on a solution and to address the budget and to address the debt,” he said. 

Unlike then-Rep. Steve Daines, whom he was running to succeed, Rosendale said he would not have voted to raise the debt ceiling to end the 2013 government shutdown, according to an AP report from the time.

“I am glad to see the government operational but it comes at a cost to our future,” he wrote on his Facebook page that October. 

Joe Williams contributed to this report.