Jeffries, speaking from the front of the Senate chamber, argued that Trump is engaging in "complete and total presidential obstruction" that is "unprecedented in American history."

"Even President Nixon, whose articles of impeachment included obstruction of Congress, did not block key White House aides from testifying in front of Congress during the Senate Watergate hearings. ...President Trump's complete and total obstruction makes Richard Nixon look like a choir boy," he said.

Once House managers and Trump's legal team debate the amendment — they can use up to two hours — the Senate will vote on calling Mulvaney.

Republicans are expected to table the amendment, effectively killing it after also shooting down three requests for Ukraine-related documents.

Mulvaney has emerged as a central figure in the impeachment investigation; House Democrats subpoenaed him as part of their inquiry, but the White House refused to take part in the months-long probe.