In an important case about presidential powers, U.S. District Judge Ketanji Brown Jackson in Washington rejected the Trump administration's legal claim that current and former senior White House officials cannot be compelled to testify before Congress.
McGahn, who left his post in October 2018, last May defied a subpoena from the Democratic-led House of Representatives Judiciary Committee for testimony. The subpoena was issued months before the House opened an impeachment inquiry in September into the Republican president's actions concerning Ukraine. The committee sued McGahn in August to try to enforce the subpoena.
Jackson's ruling concerns only McGahn's testimony. But by rejecting Trump's key legal argument for defying congressional subpoenas it could give other former and current presidential advisers a legal basis for cooperating with the ongoing House impeachment inquiry, legal experts said.
The Trump administration has refused to cooperate with the impeachment inquiry as well as other Democratic-led investigations and has directed current and former officials to defy subpoenas for documents and testimony. There are other legal fights over subpoenas seeking Trump tax and financial records.
McGahn emerged as a pivotal figure in the 448-page report completed in March by former Special Counsel Robert Mueller that detailed Russian interference in the 2016 election, through a campaign of hacking and propaganda, as well as extensive contacts between Trump's campaign and Moscow.
Mueller's report, released in redacted form in April, revealed about 10 instances in which Trump took actions aimed at impeding the investigation. Mueller did not exonerate Trump of obstruction of justice, though Attorney General William Barr, a Trump appointee, afterward decided that the president had not committed obstruction.
When the committee sued McGahn in August, it said it needed to speak with him to help lawmakers decide whether to include Trump's actions toward the Mueller investigation as part of an impeachment inquiry.
According to the Mueller report, McGahn told Mueller's team that Trump repeatedly instructed him to have the special counsel removed and then asked him to deny having been so instructed when word of the action emerged in news reports. McGahn did not carry out either instruction.
McGahn's relationship with Trump was strained by events of the Mueller investigation. Trump replaced him with Pat Cipollone, who still holds the job.
Several aides and advisers to Trump were charged in the Mueller investigation. All but two pleaded guilty, while longtime adviser Roger Stone and former campaign chairman Paul Manafort were convicted by juries. Mueller said he found insufficient evidence to charge Trump with criminal conspiracy concerning Russia.
House Democratic leaders have focused their impeachment inquiry on Ukraine, but have discussed pursuing a broader count of obstruction of Congress among any articles of impeachment - formal charges - brought against Trump. McGahn's testimony could bolster that part of their inquiry.
House passage of any articles of impeachment would lead to a trial in the Senate on whether to convict Trump and remove him from office. The Senate is controlled by Trump's fellow Republicans, who have shown little support for removing him.