Republican Sen. Mitt Romney told CNBC on Wednesday he's keeping a "completely open mind" when it comes to President Donald Trump's potential impeachment.

After initially declining to vote on a impeachment resolution, House Democrats will now vote Thursday to push forward with the public phase of their inquiry into Trump's potential abuse of power to influence the 2020 election by urging Ukraine to investigate former Vice President Joe Biden.

"I'm going to keep an open mind and I'm going to wait to make comments on any evidence," said the former presidential candidate and one-time Massachusetts governor. "I want to see the facts."

"In my view, it's time for me to stay silent on impeachment until the process is complete," Romney added on "Squawk Box."

Trump and Romney have had a rocky relationship over the past few years.

Earlier this month, Trump called Romney a "pompous ass" after the Utah senator said the president's push for foreign nations to investigate a rival was "wrong and appalling."

The pair have publicly criticized each other since Trump launched his 2016 presidential campaign, with Trump tweeting back then that Romney "was one of the dumbest and worst candidates in the history of Republican politics."

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Romney — who unsuccessfully ran an establishment campaign in 2012 against former President Barack Obama — held a news conference during the 2016 race to lambaste then-candidate Trump. The senator also been critical of Trump after the release of former special counsel Robert Mueller's report detailing Russian meddling in the 2016 U.S. election and potential obstruction of justice by the president.

Despite the back-and-forth, Trump, shortly after winning the White House in 2016, interviewed Romney as a candidate for secretary of State. The job went to Rex Tillerson, who lasted 13 months. In February 2018, Trump endorsed Romney's Senate run.

Romney was joined on "Squawk Box" on Wednesday by moderate Democratic Sen. Joe Manchin, who stressed the difference in roles between the House and the Senate.

The Constitution "is very clear," Manchin said. "It's not our job to tell the House to do their job."

He added, "Let them do what they're going to do. Let us see the evidence. We're going to be the jurors."

Manchin represents West Virginia, where Trump won nearly 70% of the vote over Hillary Clinton in 2016.

Trump has called the inquiry, which was largely spurred by his request for Ukraine leader Volodymyr Zelensky to "look into" Biden and his son Hunter, a "witch hunt" and a "scam." The president also asked Zelensky to "do us a favor" by looking into Ukraine's alleged role in interfering in the 2016 U.S. presidential election.