Presidential candidate and Montana Gov. Steve Bullock Steve BullockCourt removes Pendley from role as public lands chief On The Trail: Making sense of this week's polling tsunami McConnell locks down key GOP votes in Supreme Court fight MORE (D) on Tuesday said that Congress would have to begin impeachment proceedings against President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE if the transcripts of his calls with Ukrainian leaders were not released.

“Congress must swiftly investigate these allegations and demand the whistleblower report and any relevant transcripts of the President’s interactions with Ukrainian officials," Bullock said in a statement released by his campaign.

"If they don’t get it, I see no other option than starting impeachment proceedings," he continued.

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More than a dozen congressional Democrats have called for Trump's impeachment in recent days after new information emerged that the president withheld funds for Ukraine and floated the subject of former Vice President Joe Biden and his son during a call with the country's leader.

Bullock, who won his reelection bid in 2016 when Trump carried Montana in the presidential race, had been reluctant to get behind calls to begin impeachment proceedings against Trump.

The governor emphasized in an interview last month with Democratic strategist David Axelrod David AxelrodThe Memo: Warning signs flash for Trump on debates GOP hunts for leverage in revived COVID-19 talks Pelosi says there shouldn't be any debates between Biden and Trump MORE that voters are not concerned about the issue of impeachment and would rather focus on other issues like health care.

“In probably 30 different stops, no one asked me about impeachment,” Bullock said. “What they do talk about is, ‘Am I going to lose my rural hospital?’ ”