President Donald Trump impeached by US House of Representatives, but what's next?

The U.S. Senate will convene a formal impeachment trial of President Donald Trump early next year in a bitter and divisive climate where there's only one thing both parties agree on: keeping the proceedings relatively brief.

After Wednesday's House vote to impeach Trump on two counts of wrongdoing, the third impeachment trial in Senate history is all but assured to end with an acquittal that will leave the president in office.

But it will still be a dramatic political moment for both the president and the Democratic party to begin an election year with the biggest possible stage for Trump to formally present his defense.

So far, Trump has said he's willing to go along with Senate Majority Leader Mitch McConnell's plans for a short trial, but the president remains a wild card. He has repeatedly said he'd like to call his own witnesses and see his party mount a full-scale exoneration of him.

The trial could last for as little as two weeks, barring unexpected revelations or other drama. McConnell plans to block new witnesses sought by Democrats and he will keep nudging Trump to limit his defense on the way to his expected acquittal. On the eve of the House vote, McConnell declared he won't be an "impartial juror," even though he and the other 99 senators will swear at the start of the trial to deliver "impartial" justice.

The trial will also be the last chance for House impeachment managers to convince the American people that Trump betrayed his oath of office in his dealings with Ukraine. However House Speaker Nancy Pelosi said after the House votes Wednesday night that she was holding back for now on taking a step that would initiate the trial.

"We cannot name managers until we see what the process is on the Senate side," Pelosi said after the House votes. "This is a serious matter, even though the majority leader in the United States Senate says it's OK for the foreman of the jury to be in cahoots with the lawyers of the accused. That doesn't sound right to us."

The Democrats would need to win over four GOP senators to force McConnell to deviate from his quick-trial plan. But there's no sign that GOP moderates plan to force depositions from witnesses who have not yet testified, such as former National Security Advisor John Bolton, Acting Chief of Staff Mick Mulvaney or Secretary of State Michael Pompeo.

Senators in 2020

Even though Senate Minority Leader Chuck Schumer said he'd like to call additional witnesses, Democrats have their own reasons for keeping the trial from dragging on. The presidential primaries kick off in early February, and the party is eager to drum up excitement and attention for their 2020 bid to defeat Trump at the ballot box. Five of the candidates are in the Senate and will serve as jurors in the trial, forcing them to take lengthy breaks from the campaign trail at a crucial time in the race.

And senators in both parties want to act on other business, including the overhaul of Nafta that the House is expected to pass Thursday. The Senate is prohibited from taking up other legislation while the impeachment trial is underway.

McConnell's tight hold over the Senate GOP has allowed him to repeatedly torque Senate rules for maximum partisan advantage, especially to fill the courts with conservative judges. But now he must also contend with Trump's penchant for bombast ahead of votes that will put a number of vulnerable Senate Republicans on the spot 10 months before Election Day.

He'll have less "ball control" than usual, as he put it, with U.S. Chief Justice John Roberts presiding and just 51 votes needed to set the ground rules.

The trial will be more about the history of the moment and partisan theatrics than about swaying jurors to change the outcome. While the president needs support from just 34 senators to remain in office, McConnell hopes to keep all 53 Republicans united for acquittal.

Some Senate Republicans like Pat Toomey of Pennsylvania have called Trump's conduct inappropriate, but none have said they've seen anything worthy of impeachment.

Direct witnesses

Schumer said this week that the Senate should hear from four administration officials, including Bolton and Mulvaney, with direct knowledge of Trump's push for Ukraine to announce an investigation into the Bidens while he withheld a White House meeting and $391 million in military aid to defend against Russia.

The House tried to call these officials as witnesses but declined to wait for court orders to force them to appear. That process could have taken months, and Democrats say there's an urgent need to protect the 2020 elections.

No Republicans have endorsed Schumer's call for extra witnesses, and moderate Senators like Susan Collins of Maine and Lisa Murkowski of Alaska have publicly urged Schumer and McConnell to compromise on the procedures for a trial. McConnell proposed getting through opening arguments and senator questions before deciding whether to call more witnesses.

Collins and Murkowski are among the GOP senators that Democrats would try to court if they want to force a procedure against McConnell's will. Mitt Romney of Utah has also been a frequent Trump critic, and retiring Republicans like Lamar Alexander of Tennessee or Pat Roberts of Kansas would have an easier time splitting from their leader on procedural questions.

Senior senators like John Cornyn of Texas have dismissed the need to get new witnesses, blaming House Democrats for not going to court to enforce their subpoenas instead. And Lindsey Graham, the Senate Judiciary chairman who helped lead the impeachment of Bill Clinton 21 years ago, said he would oppose letting either side call witnesses.

One model for McConnell's maneuvering on Senate procedure could be the bitterly contentious confirmation of Supreme Court Justice Brett Kavanaugh last year. McConnell had hoped to confirm Kavanaugh a week earlier but he yielded to a few Republicans who wanted to reopen an FBI background check to look into decades-old allegations of sexual misconduct.

Back then it was Collins, Murkowski and former Arizona Senator Jeff Flake who forced McConnell's hand. The majority leader, however, successfully set a limited scope for the FBI's review and Kavanaugh ultimately won Collins' and Flake's votes.

A tentative schedule for the trial

6 January - Start of Senate trial, guidelines and other housekeeping measures finalised

7 January - Swearing in of senators as jurors and Chief Justice Roberts

9 January - House prosecutors and White House counsel each have 24 hours to present arguments