US President Donald Trump says he will formally object to Congress's impeachment inquiry even as he acknowledges House Democrats "have the votes" to proceed.
The White House is expected to send a letter to House Speaker Nancy Pelosi arguing Congress cannot conduct an impeachment investigation without first having a vote to authorise it.
The letter was expected to say the administration won't co-operate with the probe without that vote.
US President Donald Trump says he will formally object to Congress's impeachment inquiry. Source: AAP
Mr Trump said the resolution would likely pass, but he predicted it would backfire on Democrats.
"I really believe that they're going to pay a tremendous price at the polls," he said on Friday.
Mr Trump's comments came shortly before the Democrats sent an extensive request for documents from Vice President Mike Pence about his contacts with Ukraine.
Lawmakers have made Mr Trump's request for Ukraine to launch an investigation of former vice president Joe Biden the centrepiece of the probe.
They are investigating a whistleblower complaint that Mr Trump sought to use military assistance for Ukraine to push President Volodymyr Zelenskiy to investigate the 2020 Democratic hopeful.
The West Wing was set to allow a similar request for documents from the president's staff to go unfulfilled on Friday, likely forcing Democrats to make good on their threat to issue a subpoena for the records.
Nancy Pelosi announcing the House will begin a formal impeachment inquiry into US President Donald Trump Source: AAP
Late on Thursday, House of Representatives investigators released a cache of text messages that showed top US diplomats encouraging Ukraine's newly elected president to conduct an investigation linked to Biden's family in return for a high-profile visit with Mr Trump in Washington.
The release followed a 10-hour interview with one of the diplomats, Kurt Volker, who stepped down as special envoy to Ukraine after the impeachment inquiry began.
Mr Trump repeated on Friday that he was pressing Ukraine to investigate corruption, not trying to undermine Biden, who could be his 2020 presidential election opponent.
He made a related request of China, specifying Biden and his son, on Thursday.
As Republicans search for a response to the investigation, the absence of a procedural vote to begin the probe has been the main attack line against Democrats.
Ms Pelosi said the need for such a vote was unnecessary, saying the House is well within its rules to pursue the inquiry without it.
"The existing rules of the House provide House Committees with full authority to conduct investigations for all matters under their jurisdiction, including impeachment investigations," she wrote on Thursday in a letter to House Republican leader Kevin McCarthy after he, too, pressed for a floor vote.
Ms Pelosi says there's no need for a vote. Source: AP
Two days after telling reporters, "Well, I always co-operate", Mr Trump struck a different note on co-operating with the House probe.
"I don't know," he said. "That's up to the lawyers."
Democrats have warned that the Trump administration's obstruction of the investigation is itself a potentially impeachable offence.
With wires...
