WASHINGTON – Vice President Mike Pence on Tuesday refused to turn over documents requested by House Democrats as part of their impeachment inquiry.

The White House has said it will not cooperate with an investigation it considers unfair and illegitimate, a claim it bases in part on the fact that the House has not formally voted to begin impeachment proceedings.

Pence's attorney echoed that reasoning in a letter sent Tuesday to the chairmen of the investigating committees.

Counsel Matthew E. Morgan also wrote that some of the documents that Democrats had asked Pence to produce by Tuesday "are clearly not vice-presidential records."

"Please know that if the Committees wish to return to the regular order of legitimate legislative oversight requests, and the Committees have appropriate requests for information solely in the custody of the Office of the Vice President," Morgan wrote, "we are prepared to work with you in a manner consistent with well-established bipartisan constitutional protections and a respect for the separation of powers."

House Democrats did not immediately say Tuesday whether they will subpoena Pence for the documents.

They have warned the White House that failure to comply with requests for documents and witnesses can be evidence of obstruction, a potentially impeachable offense.

“The case for obstruction of Congress continues to build,” Rep. Adam Schiff, chair of the House Intelligence Committee, said at a joint news conference with House Speaker Nancy Pelosi Tuesday night.

Pelosi restated her position that a House vote is not needed under the Constitution to launch an impeachment investigation.

“There’s no requirement that we have a vote so at this time we will not be having a vote,” Pelosi said.

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Democrats are investigating any role Pence may have played in President Donald Trump's efforts to pressure the Ukrainian president to investigate former Vice President Joe Biden and his son, as well as a separate conspiracy theory related to interference in the 2016 elections.

Democrats have subpoenaed a variety of people inside and outside of the administration, including giving acting White House Chief of Staff Mick Mulvaney a Friday deadline to comply with a large documents request.

Democrats' initial Oct. 4 request to Pence followed a Washington Post report that Trump used Pence in his attempt to pressure Ukrainian President Volodymyr Zelensky to investigate the Bidens but was not conclusive on how much Pence knew about Trump's effort.

The Post said Pence's national security adviser monitored Trump's July call with Zelensky but didn't hear anything he felt should be relayed to the vice president.

CNN reported that Pence was given a transcript of call, but it's not clear if he read it.

The whistleblower complaint that sparked the impeachment inquiry by disclosing the call alleged that Trump instructed Pence to cancel plans to attend Zelensky’s May inauguration, a detail given in the context of Trump wanting to wait to see how the new leader “chose to act” in office.

Democrats requested documents related to the call and to a number of other events, including Pence's meeting with Zelensky in September when he substituted for Trump on a trip to Poland.

The White House maintains that Trump did nothing improper.

"What I can tell you is, all of our discussions internally, between the president and our team, and our contacts and my office with Ukraine, were entirely focused on the broader issues of the lack of European support and corruption," Pence said during a trip to Iowa last week.

Pence also told reporters he has no problem with the White House releasing transcripts of his conversations with Zelensky, something he said White House lawyers were reviewing.

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