Washington (CNN) Democratic Sen. Jon Tester of Montana said Monday he is open to voting for any of President Donald Trump's potential Supreme Court nominees.

"Once we get a person who the President puts forth, we are going to do our due diligence," he told CNN at the Capitol on Monday. "I am not going to disqualify anybody, nor am I going to say that I'm absolutely going to rubber stamp anybody.

The senator, who hails from a state Trump won in 2016, continued, "We are going to do our work to make sure this person understands the Constitution, understands rural issues, understands some of the issues I am concerned about like Citizens United. ... We'll meet with them after we get our information, visit them some more, eyeball to eyeball, and then will make a decision about whether to support or not."

Tester is one of the key Senate Democratic votes that Republicans might need in order to confirm Trump's Supreme Court nominee, given the narrow majority Republicans have in the chamber. Trump is expected to announce his pick on Monday evening at 9 p.m. ET at the White House.

Unlike a few other red-state Democrats, Tester was not invited to the visit the White House for the announcement. He and Trump have had a contentious relationship in the past, after Tester publicized a report that alleged inappropriate behavior by Trump's former pick to run the Department of Veterans Affairs, Ronny Jackson.

Read More