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Lawmakers return to Washington on Monday after the holiday break — and will walk right into the face-off over President Donald Trump's impeachment trial.

The House voted to impeach Trump for abuse of power and obstruction of Congress on Dec. 18, making him just the third president in U.S. history to be impeached.

But House Speaker Nancy Pelosi, D-Calif., has not yet named the case managers — essentially the members of Congress who act as prosecutors during a trial in the Senate — nor has she sent the two articles of impeachment to the Senate. The president's trial cannot get underway until she does.

Pelosi said she first wants assurances of a fair trial, and Senate Minority Leader Chuck Schumer, D-N.Y., is demanding that witnesses be allowed to testify. Senate Majority Leader Mitch McConnell, R-Ky., said he wants the issue of witnesses to be decided not now but later in the trial process, as it was during Bill Clinton's impeachment trial in 1999.

Here's where things stand and how they're likely to proceed.

What happens next?

The Clinton trial rules, which were approved in a unanimous bipartisan vote, called for the House managers to make opening statements and lay out their evidence over 24 hours, and gave lawyers for the president 24 hours to put forth their defense. When their statements were done, senators were able to question the two sides by submitting questions in writing to the chief justice of the Supreme Court, who's constitutionally mandated to preside over the trial. The chief justice read their questions aloud.

Senators were then able to vote to dismiss the case, or to continue and hear from witnesses. A motion to dismiss the Clinton case failed, and senators voted to have three witnesses give videotaped testimony. Snippets of their testimony were played on the Senate floor before closing arguments.

In the Trump trial, Democrats are concerned that the Republican majority in the Senate will toss the case against him without hearing from witnesses. Fifty-one votes would be needed to dismiss; there are 53 GOP senators.

Senate Majority Leader Mitch McConnell, a Republican from Kentucky, speaks during a news conference after a weekly caucus meeting at the U.S. Capitol on Dec. 17, 2019. Andrew Harrer / Bloomberg via Getty Images file

McConnell maintains what was fair for Clinton is fair for Trump. Democrats say there's a key difference — Clinton largely cooperated with the investigation into his conduct, even testifying before a grand jury and turning over a blood sample to the special prosecutor, Kenneth Starr.

Some moderate Republicans have indicated they may want to hear from witnesses, while McConnell has not committed to calling any. At least two Republican senators, Lisa Murkowski of Alaska and Susan Collins of Maine, have suggested they are open to having witnesses testify.

What's the holdup?

McConnell said on the Senate floor on Friday that "we can't hold the trial without the articles" of impeachment and he suggested the Senate would conduct other legislative business until Pelosi sends them. Pelosi, he complained, is "trying to dictate our process."

"That's obviously a nonstarter," McConnell said.

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Trump has railed against Pelosi for not immediately sending the impeachment articles to the Senate. "The Radical Left, Do Nothing Democrats said they wanted to RUSH everything through to the Senate because 'President Trump is a threat to National Security' (they are vicious, will say anything!), but now they don’t want to go fast anymore, they want to go very slowly," Trump tweeted. "Liars!"

But Pelosi and Schumer have given no indication they plan to back down any time soon.

"Leader McConnell reminds us today and in previous days that rather than acting like a judge and a juror, he intends to act as an executioner of a fair trial," Schumer said Friday.

If and when the two articles are sent to the Senate, the trial should begin fairly quickly. The Senate would vote on the rules and swear in Chief Justice John Roberts to preside. While the case can be dismissed by a simple majority vote, a two-thirds vote — 67 senators — is needed to convict and remove the president.

What about the prosecutors?

Pelosi would name an unspecified number of House members as case managers. (The impeachment trial of President Andrew Johnson in 1868 had had seven case managers, while Clinton's had 13.)

Pelosi's picks have yet to be revealed, but Intelligence Committee Chairman Adam Schiff, D-Calif., and Judiciary Committee Chairman Jerrold Nadler, D-N.Y., are expected to make the cut.

Freshman House Democrats also have been urging Pelosi to name Rep. Justin Amash of Michigan a case manager. Amash, an independent, quit the GOP last year after calling for the president's impeachment.

"I really believe he will elevate the substance and the optics of the trial in the Senate," Rep. Dean Phillips, D-Minn. who is leading the effort, told NBC News in a phone interview.

What about the defense lawyers?

Trump's team is expected to be led by White House counsel Pat Cipollone, who's been strategizing with McConnell and other Senate Republicans. Cipollone is the lawyer who directed all executive branch employees not to cooperate with the House impeachment inquiry.

Rudy Giuliani, Trump's personal lawyer and a key figure in the House inquiry, said last week he'd "love to try the case" himself, but added, "I don't know if anybody would have the courage to give me the case."

President Donald Trump delivers remarks at the Turning Point USA Student Action Summit at the Palm Beach County Convention Center in West Palm Beach, Florida on Dec. 21, 2019. Marco Bello / Reuters file

A source told NBC News that there have been discussions about adding famed attorney Alan Dershowitz to the Trump team. Dershowitz, whose clients have included O.J. Simpson and Jeffrey Epstein, was seen chatting with Trump at a Christmas Eve party at the president's Florida resort. Giuliani was at Mar-a-Lago during the president's vacation as well.

Rep. Mark Meadows, a close Trump ally, said GOP House members could help with the president's case, too. "If I had to pick a team, I would say you get in Jim Jordan, Matt Gaetz, John Ratcliffe,” Meadows said in an interview with former White House adviser Steve Bannon. He added that it was important for Republicans to have "an A-team" defending Trump.

What about witnesses?

Schumer has named four people that Democrats want to testify: Acting White House chief of staff Mick Mulvaney; Robert Blair, senior adviser to Mulvaney; former White House national security adviser John Bolton; and Michael Duffey, associate director for national security at the Office of Management and Budget.

All four were asked to testify during the House impeachment inquiry last year, but did not at the direction of the White House. Schumer said they all have "direct knowledge" of the administration's decision to delay nearly $400 million in aid to Ukraine.

Other possible witnesses Democrats might want to include Secretary of State Mike Pompeo and Defense Secretary Mark Esper, who both urged Trump in August to release the aid.

Sen. Marco Rubio, R-Fla., tweeted that "Democrats keep playing stupid games" and that the House passed the articles of impeachment "based on testimony of certain witnesses." "Senate has no duty to go beyond those witnesses in our trial," Rubio said.

Republicans have not released a list of possible witnesses, but have hinted that they’d like to hear testimony from Joe Biden and his son Hunter. Trump had wanted Ukraine to investigate both of them because of the younger Biden's lucrative work on the board of the Ukrainian gas company Burisma.

Some Republicans have also said they’d like to hear from the still-anonymous whistleblower who first filed the formal complaint about misconduct by the president that led to the House impeachment inquiry.

Trump tweeted last month that in addition to the Bidens, he would want Pelosi and Schiff to testify. Schiff has rejected those calls, but if he's selected as a case manager, senators could submit questions for him to answer.

How long would a trial take?

Sen. Lindsey Graham, R-S.C., a top Trump ally, has said he thinks a trial would last about two weeks.

"We'll listen to the House case, allow the president to make comments through his legal team, then we'll vote, and the sooner the better for me," Graham said after the House impeachment vote.

"I've made up my mind about the accusations."