US President Donald Trump has signalled his administration will not co-operate with an impeachment inquiry into his conduct after Democrats subpoenaed the White House about contacts with Ukraine.
In a series of tweets sent as the presidential motorcade ferried him to his Virginia golf course, Mr Trump defended his comments and lashed out at critics, including Republican Senator Mitt Romney.
Donald Trump has lashed out at critics, including Republican Senator Mitt Romney. Source: AP
"Mitt Romney never knew how to win. He is a pompous 'ass' who has been fighting me from the beginning," the president tweeted of his fellow Republican.
He also attacked the impeachment inquiry, calling it "fraud against the American people!"
The inquiry reached deeper into the White House when the House sent a letter on Friday to Mr Trump's acting chief of staff Mick Mulvaney, informing him that the White House was being subpoenaed for documents it had refused to produce.
Trump received support from Secretary of State Mike Pompeo, who dismissed questions about Mr Trump's attempts to push Ukraine and China to investigate Democratic rival Joe Biden as a "silly gotcha game".
US President Donald Trump. Source: AAP
"The president has every right to have these set of conversations," Mr Pompeo told reporters while travelling in Greece.
He insisted the administration's foreign efforts were reasonable, responsible and necessary to target graft, ensure aid is spent properly and protect American democracy.
"There has been some suggestion somehow that it would be inappropriate for the United States government to engage in that activity and I see it just precisely the opposite," he said.
It is illegal to solicit campaign help from a foreign government.
Mr Trump, who has described his conduct as "perfect", said on Friday he would formally object to Congress's impeachment inquiry, even as he acknowledged Democrats "have the votes" to proceed.
Democrats accused Mr Trump of speeding down "a path of defiance, obstruction and cover-up".
They warned that defying the House subpoena would in itself be considered evidence of obstruction and a potentially impeachable offence.
Politicians are focused on Mr Trump's efforts to push Ukraine to investigate former vice president Biden and his son Hunter.
A whistleblower complaint said Trump sought to use military assistance for Ukraine as leverage to push President Volodymyr Zelenskiy to launch an inquiry into the 2020 Democratic hopeful.
"We deeply regret that President Trump has put us - and the nation - in this position, but his actions have left us with no choice," wrote the three Democratic House chairmen, Elijah Cummings, Adam Schiff and Eliot Engel, in issuing Friday's subpoena.
The White House has planned to send a letter to House Speaker Nancy Pelosi arguing Congress cannot undertake an impeachment investigation without first having a vote to authorise it.
White House press secretary Stephanie Grisham derided the subpoena as coming from a Democratic "kangaroo court".
Democrats also have sent a separate extensive request for documents to Vice President Mike Pence, focused on his contacts with Ukraine.
Mr Pence's spokeswoman Katie Waldman dismissed the demand, saying "it does not appear to be a serious request" given its wide scope.
