Sen. Mitt Romney, R-Utah, slammed President Donald Trump for urging Ukraine and China to investigate former Vice President Joe Biden and his son, calling the appeals "appalling."

"By all appearances, the President’s brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling," Romney, the 2012 Republican presidential nominee, tweeted Friday.

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. — Mitt Romney (@MittRomney) October 4, 2019

He also mocked Trump's insistence that he is motivated by concerns about 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 wrote.

Trump was asked by a reporter earlier Friday if he'd ever asked "foreign leaders for any corruption investigations that don't involve your political opponents."

"You know, we would have to look," Trump answered.

WATCH: @EamonJavers: "Have you asked foreign leaders for any corruption investigations that don't involve your political opponents?"



President Trump: “You know, we would have to look" pic.twitter.com/H6AUH8b4qX — MSNBC (@MSNBC) October 4, 2019

Romney's comments came hours after Sen. Ben Sasse of Nebraska became the first sitting Republican senator to publicly criticize Trump for his comments on China on Thursday.

"China should start an investigation into the Bidens because what happened in China is just about as bad as what happened with Ukraine," Trump told reporters Thursday.

In a statement to the Omaha World-Record, Sasse said, “Hold up: Americans don’t look to Chinese commies for the truth. 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.”

Despite Trump's accusations of corruption, there has been no evidence of wrongdoing on the part of the former vice president or his son.

In the same statement, Sasse took aim at House Intelligence Chairman Adam Schiff, while also appearing to subtly push back against threats from the president and his personal lawyer Rudy Giuliani to sue the California lawmaker.

“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.

Trump has called Schiff "a liar" and "a fraud" who should "resign in disgrace" for how he paraphrased and characterized a phone call between Trump and the Ukrainian president, in which Trump asked his counterpart to investigate the Bidens. Schiff prefaced his remarks as saying they were "the essence of what the president communicates," but Republicans have said they were deeply misleading.

Sasse is a longtime critic of the Chinese government. While Trump tweeted his congratulations to the Chinese government on the 70th anniversary of Communist Party rule earlier this week, Sasse issued a statement decrying its history of "communist oppression."

Florida Sen. Marco Rubio, who has also spoken out strongly against the Chinese government, blew off Trump's suggestion that China investigate the Bidens. "I don't think it's a real request," he told reporters Friday morning before tweeting about it in the afternoon. "You can keep getting played,but I wont I'll focus on dignified work,collapsing families & communities, China's plan to overtake us etc," the former 2016 presidential candidate wrote.

Sasse and Romney also criticized Trump last week — Sasse after reading a whistleblower's complaint about Trump pushing for Ukraine to investigate the Bidens and Romney after a summary of the call was released by the White House showing Trump had indeed done so.

Most other Republicans shrugged the revelations off, which Romney also decried at an appearance at the Atlantic Festival.

"I think it’s very natural for people to look at circumstances and see them in the light that’s most amenable to their maintaining power, and doing things to preserve that power," he said.

Trump responded to Romney at the time by tweeting out a clip of Romney's reaction to losing the presidential election to Barack Obama in 2012.

Sasse was once a frequent Trump critic, but has taken a softer tone toward the president in recent months.

Trump even plugged Sasse's re-election effort last month, tweeting that he "has done a wonderful job representing the people of Nebraska. He is great with our Vets, the Military, and your very important Second Amendment. Strong on Crime and the Border, Ben has my Complete and Total Endorsement!"