The Democrat-led House Judiciary Committee sued a key witness in the Trump-Russia investigation Wednesday, asking a federal judge to force former White House counsel Don McGahn to testify as they explore possible impeachment proceedings against President Trump.

Rep. Jerry Nadler, a Democrat from New York, the committee chairman and longtime Trump foe, has pushed for an impeachment investigation for months even as House Speaker Nancy Pelosi of California has shown serious hesitance about the strategy, and today’s lawsuit could represent the most significant step in that direction yet. McGahn has refused to comply with a subpoena issued by Nadler’s committee earlier this year and declined to testify in front of the committee.