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(CNN) —  

Republican Sen. Mitt Romney said Sunday that it “would be troubling in the extreme” if President Donald Trump pressured Ukraine’s president to investigate former Vice President Joe Biden amid an ongoing controversy over a call Trump had with the foreign leader that was part of a whistleblower complaint.

In a tweet, Romney, who was his party’s presidential nominee in 2012, also said it was “critical” for the facts surrounding the call to emerge.

“If the President asked or pressured Ukraine’s president to investigate his political rival, either directly or through his personal attorney, it would be troubling in the extreme. Critical for the facts to come out,” Romney, who represents Utah, wrote in the tweet.

The comments from the prominent GOP senator are some of the strongest to come from his party, whose members have largely remained silent following news last week that a whistleblower complaint submitted to the Intelligence Community Inspector General was concerned in part with a July 25 call Trump had with Ukrainian President Volodymyr Zelensky. On Sunday, after Trump acknowledged that he discussed Biden during the call but maintained that the conversation was “warm and friendly,” congressional Democrats urged an investigation into the matter, with some calling for Trump’s impeachment.

CNN previously reported Trump pressed Zelensky in the call to investigate Biden’s son, Hunter, according to a person familiar with the situation. There is no evidence of wrongdoing by either Joe or Hunter Biden.

Trump previously branded criticism of the call a “Ukranian Witch Hunt,” while Biden accused the President of abusing his power to “smear” him. In comments to reporters on Sunday, the President said he hopes officials release details of the call, but slammed the whistleblower responsible for filing a complaint.

On Sunday, House Speaker Nancy Pelosi issued a strong warning to the administration, which has refused to hand over the whistleblower complaint, saying it “will be entering a grave new chapter of lawlessness” if the resistance persists. House Intelligence Committee Chairman Adam Schiff, meanwhile, said Sunday that impeachment “may be the only remedy” to Trump’s refusal to make public the complaint and phone call transcript.

Massachusetts Sen. Elizabeth Warren, a 2020 presidential candidate, and New York Rep. Alexandria Ocasio-Cortez, both Democrats, urged Congress to impeach Trump over the matter on Saturday, writing in tweets that their colleagues had an obligation to do so.