House Democrats on Friday issued a subpoena to Vice President Mike Pence for documents related to his involvement in the scandal over President Donald Trump's pressuring of the Ukrainian government to investigate one of the president's political opponents in order to access military aid.

"Pence is neck-deep in this scandal," tweeted writer Judd Legum.

The Democratic leaders of the House Oversight and Reform, Intelligence, and Foreign Affairs Committees—Reps. Elijah Cummings (Md.), Adam Schiff (Calif.), and Eliot Engel (N.Y.), respectively—made the demand in an official letter (pdf) Friday afternoon.

NEW... Dems now target PENCE pic.twitter.com/3gvhKW6to6 — Jake Sherman (@JakeSherman) October 4, 2019

By targeting Pence in the unfolding impeachment inquiry, House Democrats are opening a new front in the probe into whether the administration conditioned aid to the government of Ukrainian President Volodymyr Zelensky on Zelensky's committing to investigate Hunter Biden, son of former Vice President Joe Biden, the current frontrunner for the Democratic 2020 presidential nomination.

The committee chairmen made the demand of Pence to answer "questions about any role you may have played in conveying or reinforcing the president's stark message to the Ukrainian president" about conditioned aid.

Quite a document request for Pence from House investigators. pic.twitter.com/XsTmyhNcFA — Jennifer Bendery (@jbendery) October 4, 2019

Pence's level of involvement in the scandal was unclear until Wednesday, when reporting from The Washington Post revealed that the vice president was on the July 25 phone call to Zelensky—along with Secretary of State Mike Pompeo—the news of which set the scandal in motion. Over a month later, on September 1, Pence met with Zelensky and delivered a message:

In his meeting with Zelensky, Pence conveyed the news that hundreds of millions of dollars in U.S. aid to Ukraine was not going to be released amid concerns about the country's lagging efforts to combat corruption. He also emphasized Trump's frustration that he thought the European Union was not doing a sufficient job in helping to provide aid. A participant in the meeting said Zelensky was "crestfallen" by the news, though a second participant described the meeting as "cordial" and Zelensky as understanding of U.S. concerns.

Official transcripts from a press conference at the White House the next day show Pence refusing to answer directly a reporter's question about aid to Ukraine conditioned on investigating the Bidens.

The Post also reported that a Pence aide listened in on the call and took notes. The article was part of news that prompted the subpoena, according to the letter.

"Recently, public reports have raised questions about any role you may have played in conveying or reinforcing the president's stark message to the Ukrainian president," the letter reads.

It's unclear whether or not Pence will answer the demand. Trump plans to inform House Speaker Nancy Pelosi (D-Calif.) by way of an official letter next week that the administration will refuse any subpoenas until the House holds an official vote to open the inquiry—an unnecessary formality as the majority Democrats are almost universally united behind impeachment.