WASHINGTON – House Democrats on Friday asked Vice President Mike Pence to turn over a host of documents by Oct. 15 as part of their impeachment inquiry into President Donald Trump.

The letter was sent hours before Democrats issued a subpoena to Trump's acting chief of staff, Mick Mulvaney, demanding documents related to the president's efforts to pressure the Ukrainian president to dig up dirt on Joe Biden.

Democrats separately told Pence they're investigating any role he may have played in those efforts.

"Yesterday, you appeared to condone President Trump's efforts to press foreign powers to target the president's political opponents with baseless conspiracy theories," the chairmen of the House intelligence, foreign affairs and oversight committees wrote to Pence.

“It remains unclear to what extent you had knowledge of specific aspects of some of these events,” the letter states.

Pence, on Thursday, said that it was "worth looking into" the fact that Biden's son Hunter served on the board of a Ukrainian gas company when his father was the Obama administration's point man in Ukraine.

"And the president made it very clear that he believes ... other nations around the world should look into it as well," Pence told reporters during a trip to Arizona.

The chairmen of the House committees leading the investigation wrote in their letter to Pence that failure to comply with the three-page document request will be evidence of obstruction of the House’s impeachment inquiry and “may be used as an adverse inference against you and the president.”

Pence's spokeswoman said the letter has been sent to the White House Counsel's Office for a response.

"Given the scope, it does not appear to be a serious request but just another attempt by the Do Nothing Democrats to call attention to their partisan impeachment," spokeswoman Katie Waldman said in a statement.

The White House has indicated it will not cooperate with document requests unless and until the full House votes to authorize the impeachment inquiry, a move that Democratic leaders argue is not required.

Democrats on Wednesday had threatened to subpoena the White House if it didn't turn over by Friday a variety of documents that include any communication Pence's office had about the administration's interactions with Ukrainian President Volodymyr Zelensky.

But Friday's letter was the first sent specifically to the vice president.

The chairmen wrote in a joint letter released late Thursday that new reports had revealed that other Trump administration officials "may have been involved in the illicit effort to get Ukrainian help for the president's campaign."

The Washington Post reported Wednesday that Trump used Pence in his attempt to pressure Zelensky to investigate Biden 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 Thursday 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.

The White House maintains that Trump did nothing improper.

"Anyone that looks at the president's transcript (of the July call) will see that the president was raising issues that were appropriate, that were of genuine interest to the American people," Pence said Thursday. "I mean, the simple fact is that, you know, when you hold the second highest office in the land, it comes with unique responsibilities – not just to be above impropriety, but to be above the appearance of impropriety."

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.

During a news conference the next day, Pence was asked whether he discussed Biden with Zelensky.

“The answer is no,” Pence said.

He gave a less direct response to the question of whether he could assure Ukraine that the hold up of military assistance was not related to efforts by Trump allies to try to investigate Biden.

"The security assistance at that point had not been given to the Ukrainians," said Rep. Eric Swalwell, D-Calif., a member of the intelligence and judiciary committees. "So was (Pence) a part of this scheme that the whistleblower has described and the president has confessed to?”

Swalwell spoke to CNN after a daylong meeting with the inspector general for the intelligence community, this week's second closed-door hearing in the impeachment inquiry.

Text messages released by Democrats after Thursday's hearingshow a high level of apprehension among State Department officials that Trump had linked U.S. assistance to Ukraine to an agreement from Zelensky that he would reciprocate by taking steps that would help Trump politically at home.

Trump eventually released the military aid after a bipartisan backlash from from U.S. lawmakers in both parties.

Contributing: Bart Jansen.