U.S. District Court Chief Judge Beryl Howell rejected the claim that the impeachment process is illegitimate and ordered the Trump administration to give a House committee secret material from former special counsel Robert Mueller's report on Russian meddling in the 2016 election.
Howell said the House did not have to approve a resolution formally initiating the effort for the impeachment inquiry to be valid.
Trump's fellow Republicans have insisted the investigation was invalid because there had not been a vote in the House.
Democrats - and a few Republicans who do not oppose looking into Trump's dealings with Ukraine, the focus of the investigation - say the Constitution gives Congress wide latitude in conducting an impeachment investigation and that there is ample precedent for doing much of it out of the public eye.
Trump insists he did nothing wrong and accuses Democrats of trying to overturn the 2016 election.
Backed by most of his fellow Republicans in Congress, Trump has complained repeatedly that he is being treated unfairly, and his administration has sought to block current and former officials from testifying, causing lawmakers to issue subpoenas.
A spokesman for Senate Majority Leader Mitch McConnell said the judge's ruling did not undercut the Republicans argument. "We said it was unfair and wrong. Not unconstitutional," he said.
MORE TESTIMONY PLANNED
On Friday, the House Foreign Affairs, Oversight and Intelligence Committees issued subpoenas for Ulrich Brechbuhl, a top aide to Secretary of State Mike Pompeo, and two officials from the White House's Office of Management and Budget (OMB): acting director Russell Vought and Michael Duffey, associate director for national security programs.
The anonymous whistleblower who set off the impeachment inquiry said in a written complaint that he was told Brechbuhl had listened in on a call between Trump and Ukrainian President Volodymyr Zelenskiy in July that is at the heart of the scandal.
The inquiry focuses on Trump's request for Ukraine to investigate a domestic political rival, Joe Biden, the former vice president who is a leading contender for the Democratic presidential nomination to face Trump in the 2020 election.
Via the OMB, Trump had withheld $391 million in security assistance for Ukraine that had been approved by Congress. During the call, the Ukrainian president agreed to investigations of Biden and his son Hunter Biden's tenure on the board of a Ukrainian energy company.
U.S. law prohibits candidates from accepting foreign help in an election.
Lawmakers want Duffey to appear on Nov. 5, and Vought and Brechbuhl on Nov. 6, the committees said in a statement.
The committees have scheduled more testimony within the next week, starting with Philip Reeker, acting assistant secretary of state for European and Eurasian affairs, on Saturday.
REPUBLICAN FIREWALL
The impeachment inquiry could lead to the House approving formal charges - articles of impeachment - which would prompt a trial in the Senate on whether to remove Trump from office. The Senate is controlled by Trump's fellow Republicans, who have shown little inclination toward removing him.
Fifty, all but three, of the Republican senators have now signed on to a resolution, led by Senator Lindsey Graham, calling the impeachment process unfair.
Trump has pressured Republicans to adopt a more robust defense after two recent decisions - withdrawing U.S. troops from Syria and the now-reversed plan to hold a G7 summit at one of his resorts - drew criticism from members of his party.
Congressional Democrats, faced with stepped-up efforts by Trump's Republicans to disrupt the inquiry, are considering calling only career government employees to testify at public hearings expected to start later next month, sources said.