U.S. President Donald Trump on Saturday opened a new front in the impeachment battle that threatens his administration, blasting a prominent member of his party for criticizing his push to get foreign nations to probe a leading Democratic rival. 

The Republican president, who launched a stream of invective at Democrats and the media this week as an impeachment probe in Congress widened, 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."

In a later tweet, Trump said he was hearing that people in Utah regretted electing Romney to the Senate in 2018. "I agree! He is a fool who is playing right into the hands of the Do Nothing Democrats! #IMPEACHMITTROMNEY," 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. 

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 September 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 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 at another event 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.