Sen. Jon Tester (D-Mont.), one of the Senate’s most vulnerable incumbents, on Monday said President Biden needs to “prove” he can serve another four years after his weak debate performance, which has put scrutiny on the president’s energy and mental clarity.

“President Biden has got to prove to the American people — including me — that he’s up to the job for another four years. Meanwhile, I’ll continue to do what I’ve always done: Stand up to President Biden when he’s wrong and protect our Montana way of life,” Tester said in a statement.

Tester has sought to distance himself from the Biden administration over its handling of security at the southern border and other issues, such as allowing beef imports from Paraguay into the country.

He faces a tough reelection race against Republican Tim Sheehy in a state former President Trump won in 2016 and 2020.

Tester issued his statement ahead of the Senate’s first vote Monday afternoon after a two-week recess, during which many senators stayed quiet about Biden’s disastrous debate performance.

Other Democratic senators are taking a similarly cautious stance about whether Biden should be the party’s nominee in November.

Larkin Parker, a spokesperson for Sen. Michael Bennet (D-Colo.), said Bennet also wants Biden to reassure the American people he’s up to the task of beating Trump.

“Sen. Bennet believes President Biden must reassure the American people that he can run a vigorous campaign to defeat Donald Trump. As the Senate comes back into session this week, he plans to discuss with his colleagues the most viable path forward in this existential election,” the aide said.