Sen. Jon Tester is going after Republican ticket-splitters in his challenging reelection bid in Montana.

The three-term Democratic senator and third-generation farmer likely must win in November for his party to have any chance of defending its slim Senate majority, and he hopes Republican voters will help him do just that.

In a Republicans for Tester television ad released over the Labor Day weekend, his campaign plays up his bipartisan appeal.

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“I’m going to vote for Donald Trump, but I am voting for Jon Tester,” Wade Jones, a self-described Republican, says in the 30-second spot, first reported by Punchbowl News.

Mr. Tester is running against Republican Tim Sheehy, a businessman and former Navy SEAL who has the blessing of former President Donald Trump.

The Democrat is locked in a dogfight. But he has defied the odds before.

He won reelection in 2012 and 2018 in a ruby-red state. So, Democrats were relieved when he announced he would seek another term.

However, with Mr. Trump leading the GOP ticket, this will be Mr. Tester’s toughest fight.

Mr. Trump carried the state by more than 16 percentage points over President Biden in 2020 and more than 20 points over Hillary Clinton in 2016.

In the new ad, the pro-Tester Republicans praise his unwavering “Montana values” and his work for military veterans. They point out that Mr. Tester “got over 20 bills signed into law by President Trump” and “he stood up to Biden on the Keystone Pipeline and protected our Second Amendment rights.”

“Jon doesn’t care about party,” says Kayleigh Schaaf, another self-declared Republican. “He just does what is right for Montana.”

The latest campaign financials showed Mr. Tester has raised nearly $44 million for his reelection bid and has almost $11 million cash on hand. Mr. Sheehy has raised $14 million and has $3.2 million in the bank.

Correction: An earlier version of this story misspelled Sen. Jon Tester’s first name.