Sen. Jon Tester (D-Mont.), in the fight for his political life, is leaning on bucking his own party and President Biden to keep his Senate seat.

Why it matters: Tester's campaign in deep-red Montana could be the difference between Democrats keeping control of the Senate or relinquishing it to the GOP.

• Some of the earliest advertising from the Tester camp have elevated his splits with Biden, including an ad that said the Democrat "fought to stop President Biden from letting migrants stay in America instead of remain in Mexico."
• But Republicans are pouring money into the race and are painting Tester as a lawmaker who says one thing to Montanans and then acts differently in Washington.

The big picture: Biden is very unpopular in Montana, so keeping the president at arm's length could be pivotal to Tester's re-election bid.

• Another ad from early in the cycle highlights how Tester joined with Republicans to protect funding for gun safety and hunter education classes.
• He has also made recent moves on Capitol Hill to differ from Biden, including pushing the president to take action at the southern border and opposing new energy regulations.
• Former President Trump leads Biden by over 21 points in Montana, according to RealClearPolitics' polling averages.

Zoom in: Tester survived a close race in 2018. In that campaign, Tester highlighted his work with Trump on the trail. So far this year, Tester has not embraced the former president.

• The campaign's first ad from 2018 promoted all of the bills that Tester had worked on that were signed into law by Trump.
• Another ad from that cycle called "Trump Praise" showed Tester side-by-side with Trump.
• On the verge of a visit by Trump to the state in 2018, Tester also took out full-page ads in 14 newspapers to thank the president.

But with that angle missing from the campaign so far, Republicans tell Axios they plan to criticize Tester for embracing Trump when it was politically expedient in 2018 and not in 2024.

• National Republican Senatorial Committee communications director Mike Berg told Axios that Tester's distance from Trump would be "at the center" of the campaign against the Democrat.
• "Jon Tester pretended to like President Trump to get re-elected, then immediately turned around and began trashing him like a deranged liberal," Berg said.
• Tester also notably voted to convict Trump in both of his impeachments.

Zoom out: Tester, along with Sen. Sherrod Brown (D-Ohio), are two of the only Democrats running for Senate this year who have been so openly critical of the White House on the campaign trail.

• Both men are facing re-election fights in states that Trump won in 2016 and 2020.

The Tester campaign told Axios the lawmaker has "worked with President Trump and Republicans to help veterans, crack down on government waste and abuse, and support our first responders."

• "He stood up to President Biden by demanding action be taken to secure our border and protect Montana's way of life," Monica Robinson, a Tester campaign spokesperson, said.