President Donald Trump attacked Sen. Mitt Romney of Utah, a member of his own party, after Romney called Trump's appeals to Ukraine and China to investigate former Vice President Joe Biden and his son "wrong and appalling."

Trump claimed in one tweet Saturday that he had heard that there are people in Utah who regretted voting for Romney, who won an open Utah seat for U.S. Senate in 2018 with nearly 63 percent of the vote. Prior to the general election, he won the Republican primary by nearly 40 points.

Romney, a former governor of Massachusetts, was the Republican nominee for president in 2012. He lost to then-President Barack Obama.

Trump on Saturday appeared to call for Romney's impeachment as senator, using a hashtag in all capital letters.

"He is a fool who is playing right into the hands of the Do Nothing Democrats," Trump tweeted.

I’m hearing that the Great People of Utah are considering their vote for their Pompous Senator, Mitt Romney, to be a big mistake. I agree! He is a fool who is playing right into the hands of the Do Nothing Democrats! #IMPEACHMITTROMNEY — Donald J. Trump (@realDonaldTrump) October 5, 2019

In a subsequent post, the president retweeted a Fox News correspondent's posing the question of whether Romney would serve as the new Jeff Flake in the Republican Party. Flake, who served as a senator from Arizona until 2018, was a critic of Trump.

"Jeff Flake is better!" Trump said in response to the tweet.

Trump began his attacks on Romney earlier on Saturday with tweets in which he called Romney "a pompous 'ass' who has been fighting me from the beginning" and said Romney could have won the 2012 election if he "worked this hard on Obama."

"Sadly, he choked!" the president wrote.

Mitt Romney never knew how to win. He is a pompous “ass” who has been fighting me from the beginning, except when he begged me for my endorsement for his Senate run (I gave it to him), and when he begged me to be Secretary of State (I didn’t give it to him). He is so bad for R’s! — Donald J. Trump (@realDonaldTrump) October 5, 2019

Somebody please wake up Mitt Romney and tell him that my conversation with the Ukrainian President was a congenial and very appropriate one, and my statement on China pertained to corruption, not politics. If Mitt worked this hard on Obama, he could have won. Sadly, he choked! — Donald J. Trump (@realDonaldTrump) October 5, 2019

Trump's broadsides came a day after Romney tweeted that, "By all appearances, the President’s brazen and unprecedented appeal to China and to Ukraine to investigate Joe Biden is wrong and appalling."

The Republican senator 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 had ever asked "foreign leaders for any corruption investigations that don't involve your political opponents."

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

The barrage of tweets by Trump and Romney's comments on Friday come amid an impeachment inquiry that House Democrats opened into whether Trump used the office of the president inappropriately to pressure Ukraine to investigate former Vice President Joe Biden and his son Hunter.

Biden is a Democratic frontrunner for the 2020 election.