Sen. Mitt Romney (R-UT) speaks to members of the media after a closed briefing for Senate members May 21, 2019 on Capitol Hill in Washington, DC.

Republican Sen. Mitt Romney of Utah said Friday that President Donald Trump's explanation for asking foreign countries investigate former Vice President Joe Biden and his son "strains credulity."

The rebuke from Romney, one of the only Republicans in Congress to criticize Trump's calls for Ukraine and China to investigate his potential opponent in the 2020 election, follows Trump's insistence that he's "only interested in corruption."

"When the only American citizen President Trump singles out for China's investigation is his political opponent in the midst of the Democratic nomination process, it strains credulity to suggest that it is anything other than politically motivated," Romney said in a statement Friday.

"By all appearances, the President's brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling."

Romney TWEETS

The White House did not immediately provide a response to Romney, whose statement marks the strongest-yet Republican criticism for Trump in the ongoing controversy, which led Democrats to announce an impeachment inquiry last week.

Trump asked Ukraine President Volodymyr Zelensky in a July 25 if he can "look into" unsubstantiated allegations against Biden related to his son's position on the board of a Ukrainian natural-gas company. That call came to lawmakers' attention through a whistleblower in the CIA who submitted a complaint raising alarms about Trump's ask.

Both the complaint and a five-page memorandum of the 30-minute call were made public last week.

Romney had already signaled his discomfort with the call. "Clearly what we've seen from the transcript itself is deeply troubling," he told reporters last week.

Trump has been quick to act as his own advocate in the face of the impeachment inquiry. He has repeatedly described the call as "perfect," and has denied suspicions from Democrats that he withheld hundreds of millions of dollars in military aid to Ukraine in order to pressure that country to act on his requests.

On Thursday, Trump reiterated publicly that Ukraine "should investigate the Bidens." And he added that "likewise, China should start an investigation into the Bidens, because what happened in China is just about as bad as what happened with Ukraine."

It wasn't immediately clear what Trump believes "happened in China" related to the Bidens. Hunter Biden had Cinese business connections that have been investigated by some outlets such as The New Yorker.

Later Friday, another Republican senator — Nebraska's Ben Sasse — took issue with Trump's call for China to open an investigation.

"Hold up: Americans don't look to Chinese commies for the truth," Sasse said in a statement to the Omaha The World-Herald. "If the Biden kid broke laws by selling his name to Beijing, that's a matter for American courts, not communist tyrants running torture camps."

He also offered harsh words for House Intelligence Committee Chairman Adam Schiff, D-Calif., a leading figure in the impeachment inquiry.

"Congressman Schiff is running a partisan clown show in the House — that's his right because the Constitution doesn't prohibit clown shows, but fortunately, in the Senate, we're working to follow the facts one step at a time," Sasse said.