President Donald Trump tore into Sen. Mitt Romney on Twitter on Saturday, calling the 2012 Republican presidential nominee "pompous" a day after Romney said Trump's apparent requests of Ukraine and China to investigate a political opponent were "wrong and appalling."

"Mitt Romney never knew how to win," tweeted Trump, who has often slammed the Utah Republican for his failure to defeat President Barack Obama. He said Romney "has been fighting me from the beginning, except when he begged 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!"

On Friday, Romney derided Trump's call for China to investigate vague corruption allegations against former Vice President Joe Biden. Trump's suggestion came amid an impeachment inquiry into his attempts to get Ukrainian President Volodymyr Zelensky to investigate allegations that Biden forced a Ukrainian prosecutor to back off an energy company with ties to his son.

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Trump and his supporters have denied that his requests for foreign governments to investigate the 2020 Democratic presidential candidate were politically motivated. They have said Trump was instead focused on battling 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 tweeted.

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

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"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," Trump tweeted in response on Saturday. "If Mitt worked this hard on Obama, he could have won. Sadly, he choked!"

The president also repeated his assertion that his conversation with Zelensky was a "perfect phone call" and that the whistleblower who helped spark the impeachment inquiry was "way off" in his characterization of that was said. Although many experts and commentators have said a rough transcript of the call that was released by Trump helps corroborate the allegations against him, the president also repeated his claim that the transcript exonerated him.

Romney has been a fierce critic of Trump since the 2016 Republican primary campaign. Though Romney was not a candidate that year, he was a leading voice among Republicans who hoped to stop Trump from securing the nomination, calling him a "phony" and a "fraud" whose "personal qualities would mean that America would cease to be a shining city on a hill."

Trump responded to that criticism by calling Romney a "failed candidate" who "ran one of the worst races in presidential history" and "choked like a dog."

Their relationship improved after Trump's 2016 victory and Romney was considered a top contender for secretary of state as Trump assembled his Cabinet. And Trump backed Romney in his 2018 Senate run, tweeting that he would "make a great Senator" and that he "had my full support and endorsement."

But ahead of his swearing-in as senator, Romney wrote an op-ed in The Washington Post that said, "the president has not risen to the mantle of the office."

"Here we go with Mitt Romney, but so fast!" Trump responded. "Would much prefer that Mitt focus on Border Security and so many other things where he can be helpful. I won big, and he didn’t. He should be happy for all Republicans. Be a TEAM player & WIN!"

As to whether Romney "begged" for that endorsement, the former Massachusetts governor did boast about the president's support in a 2018 tweet.

However, Trump had relatively low popularity in Utah where he only received about 14% of the vote in the 2016 Republican caucuses, according to The New York Times. And though Trump won Utah with 45% of the vote in the 2016 general election, 21% of voters went with independent candidate Evan McMullin, who ran as a conservative alternative to Trump. Romney won the state with almost 73% of the vote in 2012.

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