US Senator and former Republican presidential nominee Mitt Romney called President Donald Trump's appeal to China and Ukraine to investigate former Vice President Joe Biden "wrong and appalling" on Twitter Friday.

Trump responded in Saturday morning tweets by writing that Romney is a "pompous 'ass'," suggesting he "begged" for the president's endorsement, and saying he "choked" during the 2012 presidential election when President Barack Obama won his second term.

The president's tweets lashing out at Romney joined his now-usual slate of attacks aimed against Democrats and the media, which have become more frenzied as the impeachment inquiry into his Ukraine dealings intensifies.

On Friday, House Democrats subpoenaed the White House and President Mike Pence, asking for documents related to Trump's July call with Ukraine President Volodymyr Zelensky that sparked talk of impeachment.

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President Donald Trump lashed out against Mitt Romney in Saturday morning tweets that called him a "pompous 'ass'" and suggesting that he "choked" during the 2012 presidential elections.

Trump's attack followed Romney's sharp criticism of the president's appeal to Ukraine and China to investigate "corruption" concerning former Vice President and current Democratic presidential candidate Joe Biden. On Twitter, Romney wrote that Trump's actions are "wrong and appalling."

During his usual morning criticisms of House Democrats and the media, which have worsened alongside the impeachment inquiry into whether Trump attempted to interfere in the 2020 election using foreign officials, Trump reiterated that his conversation with Ukraine President Volodymyr Zelensky was "very appropriate."

This photo is from 2012, when Donald Trump thought better of Mitt Romney and endorsed him. AP Photo/Gerald Herbert

Romney, who has criticized Trump ever since his 2016 campaign began, is one of a handful of Republicans who have condemned the president's behavior.

"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 in his tweet.

Trump and his personal lawyer, former New York City Mayor Rudy Giuliani, have asserted that Biden and his son Hunter have engaged in "corruption" concerning Hunter's business dealings in Ukraine and China, and Biden's role of Vice President at the time. There is currently no evidence that either Biden engaged in wrongdoing.

On Thursday, Trump openly stated his appeal to China and Ukraine to investigate the Bidens. On Friday, House Democrats subpoenaed the White House and Vice President Mike Pence for "stonewalling" its inquiry, which stemmed from a whistleblower complaint about Trump's July call with Zelensky, in which he suggested that foreign aid for Ukraine would be withheld unless Zelensky aided Trump's campaign interests.

Read more: Just a handful of Republican lawmakers have rebuked Trump's calls for Ukraine and China to interfere in the US election

In his response to Romney, Trump wrote "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!"

In another tweet, the president said "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!"

In February 2018, Trump tweeted his endorsement of Romney for the position he now holds as a US senator in Utah. Romney also previously stated that he and Trump were on the same page, policy-wise, but that he would speak out against the president if he felt it was needed.

Axios reports that Trump plans to send a letter to House Speaker Nancy Pelosi arguing that the White House can ignore the House Democrats' subpoena and any requests for documents and testimony until the full House votes to approve an impeachment inquiry.