WASHINGTON (Reuters) - The senior U.S. Senate Democrat called on Sunday for the Republican-led chamber to demand testimony from four current and former White House aides in a trial expected early next year on whether to remove President Donald Trump from office.

Senate Democratic leader Chuck Schumer urged formal legal demands for testimony from White House acting Chief of Staff Mick Mulvaney, former national security adviser John Bolton, Mulvaney aide Robert Blair and budget official Michael Duffey.

Schumer made his appeal in a letter to Republican Senate Majority Leader Mitch McConnell, who said last week he was coordinating with the White House and has raised the prospect of a short impeachment trial in which no witnesses would be called.

Democrats in the House of Representatives, where they have a majority, also sought testimony from the four men in their inquiry into whether to impeach Trump over his effort to pressure Ukraine to investigate his Democratic political rival Joe Biden. Mulvaney, Blair and Duffey all defied subpoenas from a House committee and Bolton was not subpoenaed.

Trump is expected this week to become the third U.S. president to be impeached when the House votes on charges that he abused his power by asking Ukraine to investigate Biden, a leading contender to oppose him in next year’s U.S. presidential election, and obstructed Congress’ investigation of the matter.

That would set up a trial in the Senate, which is unlikely to vote to remove the Republican president from office.

The obstruction charge against Trump is based partly on White House directives to officials not to cooperate with the impeachment inquiry.

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Trump has denied wrongdoing and called impeachment “a hoax.”

Democrats accused the president of endangering the U.S. Constitution, jeopardizing national security and undermining the integrity of the 2020 election by asking Ukrainian President Volodymyr Zelenskiy in a July phone call to investigate Biden, a former U.S. vice president, as well as his son Hunter Biden.

In his letter, Schumer also asked for the Senate to subpoena a “limited set of documents” that he believes would shed light on Trump’s decision to delay the release of $391 million in U.S. aid to Ukraine and the president’s desire for Ukraine to announce investigations into the Bidens.

‘TIMELINE HAS NOT CHANGED’

“The trial must be one that not only hears all of the evidence and adjudicates the case fairly; it must also pass the fairness test with the American people,” Schumer wrote.

He also laid out a timetable for the expected Senate trial, suggesting that pretrial proceedings take place on Jan. 6, and that senators and U.S. Chief Justice John Roberts, who will preside, be sworn in the next day.

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Trump has made the Democratic-led impeachment inquiry a rallying cry for his supporters as he attempts to win re-election next November.

A McConnell spokesman did not address the substance of Schumer’s requests, but said the Senate majority leader “has made it clear he plans to meet with Leader Schumer to discuss the contours of a trial soon. That timeline has not changed.”

The White House did not immediately respond to a request for comment on Schumer’s letter.

Asked on Friday whether he wanted a long or short trial, Trump was not clear about his preference.

“I’ll do whatever I want. Look ... we did nothing wrong. So I’ll do long, or short,” he told reporters. “It doesn’t matter.”