WASHINGTON (Reuters) - The U.S. Senate remained at a stalemate on Friday over how to proceed with the impeachment trial of President Donald Trump, as the chamber’s leaders wrangled over whether White House aides will be called as witnesses and the top Democrat appealed to a handful of Republicans who could help break the impasse.

After a two-week holiday recess, there was still no clarity about when Trump’s impeachment trial might begin.

Senate Majority Leader Mitch McConnell, a Republican, said that in any case the trial could not start without the articles of impeachment, which House Speaker Nancy Pelosi has not yet sent to the Senate.

The Democratic-controlled House of Representatives voted in December to impeach Trump for pressuring Ukraine to investigate former Vice President Joe Biden, a potential rival in the 2020 presidential election.

A trial would be held in the Senate, and Trump is expected to be acquitted by the Republican-controlled chamber. But McConnell and Senate Democratic Leader Chuck Schumer have been at loggerheads since late last year over how it should be conducted.

McConnell said on Friday that once the Senate receives the articles of impeachment from the House, it could start the trial and resolve the dispute over witnesses “mid-trial.” He said this would follow the precedent set in former President Bill Clinton’s impeachment trial two decades ago. Clinton, a Democrat, was acquitted by the Senate.

“Just like 20 years ago,” McConnell said, “we should address mid-trial questions such as witnesses after briefs, opening arguments, senator questions and other relevant motions.”

SETTING ‘A TRAP’

Schumer, speaking after McConnell, said the majority leader was trying to set a trap by waiting to consider witnesses until after opening presentations.

Night falls over the U.S. Capitol Dome, as members of the Republican-controlled U.S. House of Representatives deal with a budget showdown with the Democratic-controlled U.S. Senate, and a possible government shutdown in Washington, September 30, 2013. REUTERS/Jonathan Ernst

By that time, Schumer said, McConnell would want to wrap things up and would accuse Democrats of wanting to “drag the whole affair out” by calling witnesses. The witnesses Democrats want to call have not previously testified - unlike in the Clinton trial, Schumer said.

He has asked for testimony from Mick Mulvaney, the acting White House chief of staff, along with former national security adviser John Bolton and two other White House aides.

“So if we don’t get a commitment up front that the House managers will be able to call witnesses as part of their case, the Senate will act as little more than a nationally televised meeting of the ‘Mock Trial Club,’” Schumer declared.

In an appeal to Republicans who may have concerns about McConnell’s stance, Schumer also noted that a decision on the parameters of the trial “ultimately rests with a majority of the senators in this chamber.”

Republicans have a 53-seat majority in the Senate, where 51 votes are needed to pass a set of rules for the Trump trial.

At least two Republican senators, Lisa Murkowski and Susan Collins, expressed concerns over the holiday break about McConnell’s approach to the trial after he said he was acting in “total coordination” with the White House and would not be an impartial juror.

The actual impeachment trial in the Senate would need a two-thirds majority vote for a conviction, requiring more than 20 Republicans to break with their party to remove the president.

Schumer said Trump administration correspondence released this week bolstered Democrats’ case that Trump withheld military aid from Ukraine in an effort to pressure Kiev to investigate the Bidens.

The two Senate leaders spoke after a U.S. air strike in Baghdad killed Qassem Soleimani, commander of Iran’s elite Quds Force and architect of its growing military influence in the Middle East. The attack was authorized by Trump, and Iran has promised harsh revenge.

Clinton ordered four days of bombing on Iraq in 1998 as he was facing an impeachment vote in the House. Those airstrikes delayed the vote, but did not prevent it.