Democrats subpoena White House for documents on Trump's efforts with Ukraine

The impeachment investigation into U.S. President Donald Trump widened on Friday into a constitutional battle between the executive branch and Congress, as Democrats subpoenaed White House officials and the president signalled his administration would not co-operate.

Trump said he would formally object to the impeachment investigation, even as he acknowledged that House Democrats "have the votes" to proceed.

Democrats warned that Trump is "on a path of defiance, obstruction and coverup" and said defying their subpoena would be considered "evidence of obstruction," potentially an impeachable offence.

The White House was expected to send a letter to House speaker Nancy Pelosi arguing that Congress could not mount its impeachment inquiry without first having a vote to authorize it. The letter was expected to say the administration wouldn't co-operate with the probe without that vote.

Trump said the resolution would likely pass the House, 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.

Three Democratic chairs — Reps. Elijah Cummings, Adam Schiff and Eliot Engel — warned Trump in a letter accompanying their subpoena. 

"Speaker Pelosi has confirmed that an impeachment inquiry is underway, and it is not for the White House to say otherwise," the letter said.

Trump's comments came shortly before Democrats sent an extensive request for documents to Vice-President Mike Pence about his contacts with Ukraine.

The West Wing was set to allow the similar request for documents from the president's staff to go unfulfilled Friday, likely forcing Democrats to make good on their threat to issue a subpoena for the records.

Pence spokesperson Katie Waldman dismissed the new demand for documents, saying that given its wide scope, "it does not appear to be a serious request."

Democrats have made Trump's request that Ukraine investigate former vice-president Joe Biden the centrepiece of the impeachment probe.

A whistleblower complaint said that Trump sought to use military assistance for Ukraine to push President Volodymyr Zelensky to investigate Biden, who is currently running to be the Democratic Party's 2020 presidential candidate. 

No clear-cut procedure for impeachment inquiry

When Pelosi recently announced that the House was initiating the inquiry, she didn't seek the consent of the full chamber, as was done for impeachment investigations into former presidents Richard Nixon and Bill Clinton.

But it is underway, and at a rapidly escalating pace.

Late Thursday, House investigators released a cache of text messages that showed top U.S. diplomats encouraging Ukraine's newly elected president to conduct an investigation linked to Biden's family in return for granting a high-profile visit with 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 had begun.

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 Hunter — on Thursday.

As Republicans search for a response to the investigation, the absence of a procedural vote to begin the probe has been a main attack line against Democrats.

Pelosi swatted the need for such a vote back as 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," Pelosi wrote Thursday in a letter to House Republican leader Kevin McCarthy after he, too, pressed for a floor vote.

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Pelosi has sought to avoid a vote on the impeachment probe for the same reason she resisted — for months — liberal calls to try to remove the president: It would force moderate House Democrats to make a politically risky vote.

The White House, meanwhile, is trying to force the question on Democrats, as it seeks to raise the political cost for their impeachment investigation and to animate the president's supporters ahead of the 2020 election.

Trump allies have suggested that without a formal vote, the House is merely conducting standard oversight, entitling lawmakers to a lesser level of disclosure from the administration. The Justice Department raised similar arguments last month, though that was before Pelosi announced the impeachment investigation.

Two days after telling reporters, "Well, I always co-operate," 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 office. The administration was expected to miss various deadlines Friday to comply with House investigators' requests for documents.

There's no clear-cut procedure in the U.S. Constitution for initiating an impeachment inquiry, leaving many questions about possible presidential obstruction untested in court, said Allan Lichtman, a history professor at American University.

"There's no specification in the Constitution in what does and does not constitute a more formal impeachment inquiry or investigation," he said.

Rudy Giuliani, the president's personal attorney, dismissed the entire premise of the impeachment inquiry.

"The president was not tasking Ukraine to investigate a political opponent," Giuliani told The Associated Press on Thursday. "He wanted an investigation into a seriously conflicted former vice-president of the United States who damaged the reputation of the United States in Ukraine."