Vulnerable Sen. Jon Tester, D-Mont., is continuing to defend his decision not to endorse in the 2024 presidential race ahead of his tough re-election, but the Democrat's opponent suggests his "record" signals support for Vice President Kamala Harris.

Tester recently revealed that he would not be making an endorsement in the presidential race in order to focus on his re-election, despite being a key player in Harris' recruitment to the Senate.

Tester served as chair of the Democratic Senatorial Campaign Committee (DSCC) from 2015 to 2017, leading the efforts of the campaign arm to elect Democrats to the chamber when Harris first announced her senatorial aspirations. And after Harris launched her California Senate bid, the Los Angeles Times reported in January 2015 that, according to an adviser, Tester was one of the players who encouraged her to run.

Asked again about withholding an endorsement, Tester's response was that he would work with or hold accountable whichever presidential candidate gets elected.

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Jon Tester

Montana Democratic Sen. Jon Tester is seeking a fourth term. (William Campbell/Getty Images)

"Unfortunately, my opponent would rather talk about a national race that will be decided by the people of Montana. Whoever gets elected to the U.S. president, I can guarantee you one thing, just like all previous ones, I’ll work with them when they’re doing good things for Montana, and I’ll hold them accountable when they’re not," Tester told MTN News in an interview.

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Tester endorsed Harris when she was selected as President Biden's 2020 running mate. "My friend @KamalaHarris is a proven fighter and an excellent pick for Vice President. As VP, I'm confident she will continue to fight for working families across this country. Looking forward to supporting her and @JoeBiden in November," Tester wrote in an August 2020 post on X, formerly known as Twitter.

Harris in Michigan

Vice President Kamala Harris poses during the "Unite for America" livestreaming rally in Farmington Hills, Michigan, on Sept. 19, 2024. (Saul Loeb/Getty Images)

Montana Republican nominee and Navy SEAL Tim Sheehy, who endorsed former President Trump in the presidential race, suggested that Tester's voting record is his endorsement for Harris.

"Jon Tester votes with Kamala Harris 95% of the time. His voting record is his endorsement of the failed Harris agenda, and every time Montanans needed him to stop the insanity in Washington, he was the deciding vote to further her progressive liberal agenda," a spokesperson for Sheehy for Montana told Fox News Digital. 

"That same agenda has made costs for things like groceries and gas skyrocket, it’s brought us record high inflation, and has given us a wide-open southern border that has brought crime and deadly drugs into our communities. Tester also voted to allow benefits for illegal immigrants paid for by the American taxpayer – a hallmark of the Harris agenda. We can’t afford to re-elect Jon Tester because his voting record in Washington proves he’s changed and is and will continue to support the radical America Last agenda."

Tester is running for re-election in red state Montana, a race that could likely determine which party controls the chamber, and recent polling and political forecaster predictions suggest that Sheehy is currently leading the race just weeks before Election Day.

Tim Sheehy speaks on Day 2 of the Republican National Convention

Montana Senate candidate Tim Sheehy speaks at the Republican National Convention in Milwaukee, Wisconsin, July 16, 2024. (Mike Segar/Reuters)

A recent AARP survey found that Sheehy is leading by six percentage points in a head-to-head matchup against Tester. His lead against Tester widened to eight points in a multi-candidate field that included the state's Green Party and Libertarian candidates.

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The Cook Political Report, an independent nonpartisan elections handicapper, recently shifted the race from "toss-up" to "lean Republican," while Sabato's Crystal Ball at the University of Virginia's Center for Politics moved the race to "leans Republican."

Tester's campaign did not respond to Fox News Digital's request for comment at the time of publication.