The Republican president, who launched a stream of invective at Democrats and the media as an impeachment probe in Congress widened this week, tweeted that U.S. Senator Mitt Romney was a "pompous 'ass' who has been fighting me from the beginning."
Romney, who lost the 2012 election to Democratic incumbent President Barack Obama, criticized Trump on Friday for asking China to investigate former Vice President Joe Biden, who is seeking the 2020 Democratic presidential nomination.
The Utah Republican said Trump's appeal to China was "wrong and appalling."
"If Mitt worked this hard on Obama, he could have won. Sadly, he choked!" Trump wrote.
Trump also defended again as "appropriate" the July 25 phone call with Ukrainian President Volodymyr Zelenskiy that triggered the impeachment inquiry in the Democratic-controlled House of Representatives, and said his call for China to investigate Biden and his son Hunter was linked to corruption, not politics.
House Democrats are examining whether there are grounds to impeach Trump based on a whistleblower’s complaint that said he asked Zelenskiy to help probe the Bidens. Hunter Biden was on the board of a Ukrainian natural gas company for several years.
There is no evidence of wrongdoing by either the former vice president or his son.
Secretary of State Mike Pompeo, a Trump ally, said on Saturday the State Department had issued an initial response to a request by the House Foreign Affairs Committee for documents tied to U.S. contacts with the Ukrainian government. The panel issued Pompeo a subpoena on Sept. 27.
"The State Department sent a letter last night to Congress, which is our initial response to the document request. We will obviously do all the things we are required to by law," Pompeo said in a news conference in Greece. He did not elaborate on the contents of the letter.
An official from the House Foreign Affairs Committee said in a statement to Reuters that Pompeo had "failed to meet the deadline to produce documents required by the subpoena," but confirmed the State Department had contacted the three House committees involved in the probes.
"We hope the department will cooperate in full promptly," said the official, who spoke on the condition of anonymity.
Pompeo, who has acknowledged he listened to the July 25 call between Trump and Zelenskiy, objected last week to efforts by Democrats to obtain depositions from current and former department officials.
Asked on Saturday if there were any red flags in the Ukraine saga that required further investigation, Pompeo said he did not think the audience was interested in hearing about the issue.
"This is what’s wrong, when the world doesn’t focus on the things that are right, the things that matter, the things that impact real people’s lives and instead you get caught up in silly gotcha game?" he said on the last leg of his Europe trip.
He also addressed a notion Trump has long held that Ukraine interfered in the 2016 U.S. presidential election, a theory that runs counter to the U.S. intelligence community's findings that it was Russia that meddled in the campaign.
Pompeo said "governments have an obligation and indeed a duty to make sure that elections happen with integrity, without interference from any government, whether that’s the Ukrainian government or any other."
Lawmakers are looking at whether Trump jeopardized national security and the integrity of U.S. elections for personal political gain. Trump has called the impeachment investigation a "hoax" and accused the media and Democrats of corruption.
House Democrats subpoenaed the White House for documents on Friday. White House spokeswoman Stephanie Grisham said the subpoena "changes nothing" and that it would be ultimately shown that Trump did nothing wrong.
The probe could lead to approval of articles of impeachment - or formal charges - against Trump in the House. A trial on whether to remove Trump from office would then be held in the Republican-controlled Senate.
A two-thirds majority of the senators present would be needed to oust Trump, which means 20 Republicans would have to jump ship if all the Democrats and the two independents who caucus with Democrats form a united front.
Although there are no visible cracks in Trump's Senate firewall so far, Romney's willingness to step out of formation and criticize Trump over his calls for foreign investigations of a political opponent could act as a catalyst for others.Romney savaged Trump during the 2016 campaign, calling him a "con man" who was unfit for the presidency, but the two men later set aside their differences. Trump endorsed Romney's political comeback last year.