WASHINGTON — House Democrats filed a lawsuit Wednesday seeking to compel testimony from President Trump’s former White House counsel Don McGahn, calling him “the most important witness” in their investigation into whether to impeach Trump.

Now they’re hoping a judge in Washington D.C. will agree they need to hear from McGahn, whose testimony was key to Special Counsel Robert Mueller’s report on Trump’s ties to Russia and potential obstruction of justice.

In a lawsuit filed Wednesday, House Democrats dropped some of their most blunt language yet about their current stance on impeachment.

“The Judiciary Committee is now determining whether to recommend articles of impeachment against the President based on the obstructive conduct described by the Special Counsel,” Democrats wrote in the filing. “But it cannot fulfill this most solemn constitutional responsibility without hearing testimony from a crucial witness to these events: former White House Counsel Donald F. McGahn II.”

Previously, they’ve said they’re running an “impeachment investigation,” which is not quite the same thing as opening an official “impeachment inquiry” — although, legally speaking, they’ve insisted that it is.

On Wednesday, they called McGahn “the most important witness, other than the President, to the key events that are the focus of the Judiciary Committee’s investigation.”

House Democratic aides said they’re hoping a judge will expedite the request to force McGahn, who has been ordered by Trump to ignore a House subpoena, into appearing soon.

But McGahn doesn't believe he witnessed any law-breaking in the White House, and will only appear before Congress if a judge orders him to, his lawyer said in a statement to VICE News on Wednesday.