US President Donald Trump opened a new front in the impeachment battle that threatens his administration, blasting a prominent member of his party for criticising 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 US Senator Mitt Romney was a "pompous 'ass' who has been fighting me from the beginning".
Mr Romney, who lost the 2012 election to Democratic incumbent President Barack Obama, criticised Mr 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 Mr Trump's appeal to China was "wrong and appalling."
In a later tweet, Mr Trump said he was hearing that people in Utah regretted electing Mr 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," Mr Trump wrote.
Mr Trump also defended again as "appropriate" the 25 July 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 Mr Biden and his son Hunter was linked to corruption, not politics.
House Democrats are examining whether there are grounds to impeach Mr Trump based on a whistleblower's complaint that said he asked Mr 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.
Pompeo responds to subpoena
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 US contacts with the Ukrainian government. The panel issued Mr Pompeo with a subpoena on 27 September.
"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," Mr 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 Mr 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.
Mr Pompeo, who has acknowledged he listened to the 25 July call between Mr Trump and Mr Zelenskiy, objected last week to efforts by Democrats to obtain depositions from current and former department officials.
He also addressed a notion Mr Trump has long held that Ukraine interfered in the 2016 US presidential election, a theory that runs counter to the US intelligence community's findings that it was Russia that meddled in the campaign.
Mr 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 Mr Trump jeopardised national security and the integrity of US elections for personal political gain. Mr Trump has called the impeachment investigation a "hoax" and accused the media and Democrats of corruption.
Rudy Giuliani, Mr Trump's personal lawyer, told Reuters in a phone interview on Saturday that the president in the 25 July call had not offered his Ukrainian counterpart any "quid pro quo" for his cooperation in probing Biden and his son.
"One thing clear about the conversation is, there's no quid pro quo," said the former New York mayor, who has emerged as a central figure in the scandal. The House Intelligence Committee has issued a subpoena to Mr Giuliani, setting an 15 October deadline for him to turn over documents related to his work with Ukrainians on Mr Trump's behalf.
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 Mr 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.
Although Mr Trump's Senate firewall has no visible cracks so far, Mr Romney's willingness to step out of formation and criticise Mr Trump over his calls for foreign investigations of a political opponent could act as a catalyst for others.
Mr Romney savaged Mr Trump during the 2016 campaign, calling him a "conman" who was unfit for the presidency, but the two men later set aside their differences. Mr Trump endorsed Mr Romney's political comeback last year.