A federal judge ruled late on Monday that former White House counsel Donald McGahn must testify before the impeachment probe into Donald Trump, a decision that could have major implications for the U.S. president.

The court decision, compelling McGahn to comply with a congressional subpoena, could lead to fresh attempts to force other key players, like former national security adviser John Bolton and acting Chief of Staff Mick Mulvaney, to testify.

The chair of the House Intelligence Committee, Adam Schiff, revealed on Monday that a report that could lead to impeachment charges against Trump will be published in December. He also said the report would include a catalog of White House refusals to cooperate with the inquiry.

What did the judge rule?

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.

"Executive branch officials are not absolutely immune from compulsory congressional process – no matter how many times the executive branch has asserted as much over the years – even if the president expressly directs such officials' noncompliance," Jackson wrote, adding that "this result is unavoidable as a matter of basic constitutional law."

"Presidents are not kings," she added in her decision. "No one, not even the head of the Executive branch, is above the law." She said Congress has the power to subpoena any of the president's advisers, whether they are involved in domestic policies or sensitive national security issues. 

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. An appeal is expected. 

What are the implications?

By rejecting Trump's key legal argument for defying congressional subpoenas, it could give other former and current presidential advisers, like Bolton, 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's tax and financial records.

How about Rudy Giuliani?

Rudy Giuliani, Trump's personal lawyer and a pivotal player in the president's dealings with Ukraine, is another figure whose testimony the Democrats would welcome.

He is alleged to have led a shadow campaign to push Ukraine into investigating the Joe Biden, a political rival of the president, and Hunter Biden, but reporting on Monday suggested the former New York City mayor is also at the center of a broad investigation by the FBI and the Southern District of New York.

The Wall Street Journal and the Washington Post reported that potential charges against Giuliani and his associates include obstruction of justice, fraud, and money laundering.

Any good news for Trump?

The Supreme Court on Monday gave Trump a boost by extending its hold on a lower court ruling that required his longtime accounting firm to hand over his financial records to a Democratic-led House of Representatives committee.

The unsigned order will remain in effect until after the Supreme Court decides whether to hear Trump's appeal of the lower court ruling that directed Mazars LLP to comply with the subpoena for the records.

(With input from Reuters)