The U.S. Capitol in Washington
FILE - The U.S. Capitol in Washington is shrouded in mist, Dec. 13, 2019, at the end of an acrimonious week of partisan disputes in the House Judiciary Committee, which approved two articles of impeachment against President Donald Trump.

CAPITOL HILL - More than a week after the U.S. House of Representatives impeached President Donald Trump on charges of abuse of power and obstruction of Congress, the Senate remains deadlocked over how to proceed with a trial. 
 
While Senate Majority Leader Mitch McConnell of Kentucky says he wants to start the trial as soon as lawmakers return from the holiday recess in January, Democrats are demanding an agreement in advance to subpoena witnesses and documents during the proceeding. 
 
In sparring over how to move forward, both sides are invoking the 1999 impeachment trial of former President Bill Clinton, accusing each other of departing from what they view as precedents set by that trial. And congressional Democrats are outraged over McConnell's vow to coordinate the trial in lockstep with the White House and do everything he can to acquit Trump.    
 
Calling impeachment a "political process," McConnell has said he won't be an "impartial juror."  This, critics say, flies in the face of an oath of impartiality that all senators must take before the trial begins.     
 
To formally launch a trial, the 100-member Senate must pass a resolution with a simple majority authorizing the proceeding. But in order to do that, the House must first transmit the articles of impeachment. 

FILE - House Speaker Nancy Pelosi of California holds the gavel as the House votes on articles of impeachment against President Donald Trump by the House of Representatives at the Capitol in Washington, Dec. 18, 2019.

House Speaker Nancy Pelosi of California, however, says she refuses to deliver the documents and select impeachment prosecutors "until we know what sort of trial the Senate will conduct." McConnell is refusing to agree to any of that in advance and cites the 1999 Clinton trial as a precedent. 
 
What happened during the Clinton trial? 
 
The Clinton trial, the second of its kind in American history, began on Jan. 7, 1999, more than two weeks after he was impeached on charges of perjury to a grand jury and obstruction of justice with regard to a sex scandal investigation. 
 
The trial began after an agreement was struck between Republicans who controlled the Senate and the Democratic minority.    
 
Then two more resolutions governing the trial followed. 
 
First came a unanimously approved resolution on Senate trial procedures. It gave both sides equal time — up to 24 hours — to present their arguments, while giving senators who served as jurors up to 16 hours to question the parties. 
 
Then, more than halfway through the trial, the Senate approved, largely along party lines, a Republican plan for deposing witnesses. The witnesses — White House intern Monica Lewinsky, Clinton aide Sidney Blumenthal and Washington lawyer Vernon Jordan — provided closed-door depositions. Videotaped portions of those depositions were shown to the Senate. In the end, however, the weight of the witness testimony failed to persuade the Senate to convict Clinton. 
 
What do Democrats want? 
 
The House impeachment inquiry into Trump was launched after an anonymous intelligence community whistleblower accused the president of pressing Ukraine's president to investigate Trump's political rival, Joe Biden, and Biden's son, as well as a debunked theory that Ukraine interfered in the 2016 U.S. presidential election. 
 
While more than a dozen witnesses were deposed during the inquiry, Trump prevented other high-ranking officials from testifying or providing documents. Democrats say the officials who were barred from talking have firsthand knowledge of what happened and they want to hear from them. 

FILE - Senate Minority Leader Chuck Schumer of New York leaves after speaking at a news conference, Dec. 16, 2019, on Capitol Hill in Washington.

In a letter to McConnell last week, Senate Minority Leader Chuck Schumer of New York identified four such witnesses, including acting White House Chief of Staff Mick Mulvaney and former national security adviser John Bolton. In addition, he demanded "a limited set of relevant documents" that shed light on Trump's alleged demand for political investigations as well as the administration's withholding of a White House meeting and military aid eagerly sought by Ukraine. 
 
What do Republicans want? 
 
Republicans say they want to follow the Clinton model and open the proceeding in January, while leaving the decision to call witnesses to mid-trial. 

"What is good enough for President Clinton is good enough for President Trump," McConnell said on the TV program Fox & Friends on Monday. 
 
While insisting that "we have not ruled out witnesses," McConnell gave no indication that he'd agree to the four witnesses sought by the Democrats. 
 
What does Trump want? 
 
After initially favoring a swift trial, Trump has reportedly come around to embracing a more full-blown proceeding that will fully exonerate him. 
 
Trump previously raised the prospect of calling Pelosi, House Intelligence Committee Chairman Adam Schiff, the Bidens and "many more" to testify, while accusing Democrats of desperately seeking witnesses to strengthen what he said was their otherwise weak case. 

FILE - Senate Majority Leader Mitch McConnell of Kentucky speaks with reporters on Capitol Hill in Washington, Dec. 19, 2019.

"The only way to make this work is to mount some kind of public pressure to demand witnesses, but McConnell has the votes and he can run this trial any way he wants to," Trump tweeted on Tuesday. 
 
What does all this mean for the trial? 
 
Until it receives the impeachment articles, the Senate can't formally begin a trial, leaving the proceedings in limbo. McConnell is under increasing pressure to cut a deal if only to protect the Senate's institutional credibility. But given the existing partisan divide, few expect senators to agree to a set of rules akin to those of the Clinton trial. 
 
"The problem is that this time around, the Republicans in particular are much, much more obstreperous than the Democrats were in defending Clinton," said Jeffrey Tulis, a professor of political science at the University of Texas at Austin. 
 
How long will the trial take? 
 
An agreement on witnesses will largely determine the length of the trial. For the sake of comparison, the 1868 trial of President Andrew Johnson lasted about 80 days while the Clinton trial went on for a little over a month. McConnell has said he hoped the trial would conclude after "not too lengthy a process."