Democrats, weighing witnesses, plan to launch impeachment trial by end of week, sources say

House Speaker Nancy Pelosi is expected to send the article of impeachment against President Donald Trump to the Senate later this week, sources familiar with the matter tell ABC News -- a move that could kick off formal proceedings the next day and opening arguments on the Senate floor the following week.

The timing of formal transmission from the House to the Senate is significant, as the Constitution dictates that the trial begins at 1 p.m. the following day.

Pushing that procedural step back until after President-elect Joe Biden takes office -- back to Thursday or Friday -- would also give his administration at least a day or two to gain its footing as the Senate begins the balancing act of putting Trump on trial while starting to take up Biden's agenda.

Drew Hammill, a spokesman for Pelosi, declined to comment on the timeline.

"Stay tuned," he told ABC News.

Democrats are expected to assume control of the Senate on Wednesday, following the swearing-in of incoming Georgia Sens. John Ossoff and Raphael Warnock. Any move to begin impeachment proceedings is also contingent upon Senate leaders hashing out rules to govern the trial on the Senate floor.

In near-daily meetings, Democratic impeachment managers and lawyers have been discussing how to best make their case to the narrowly divided Senate -- focusing on the seventeen Republicans required to convict Trump and the desire to bar him from seeking office in the future. The vote to bar Trump from holding any elected office would require a simple majority, following a vote to convict from two-thirds of the Senate.

"I don't think anybody would seriously argue that we should establish a precedent where every president on the way out the door has two weeks or three weeks or four weeks to try to incite an armed insurrection against the union or organize a coup against the union," Rep. Jamie Raskin (D-Md.), the lead manager, said on CNN on Sunday, arguing that Trump needed to be barred from a future presidential run.

Democrats, who control the House and in just days will take control of the Senate, are also planning the specifics of the trial -- how long it will last, and whether they should call any witnesses or request documents from those involved in the planning or response to the Jan. 6 rally at which Trump is accused of inciting his supporters to attack the Capitol.

Adding witnesses could bolster their case, but it could also add days to the proceedings as Biden seeks to gets his cabinet nominees confirmed and works to swiftly pass additional coronavirus relief legislation.

"Witnesses are always in order, even when you're talking to others who were also witnesses," Ambassador Norm Eisen, who served as a counsel to Democrats on the House Judiciary Committee during Trump's first impeachment, told ABC News. "You need to establish the order of proof. As painful as it is, you need to put some evidence in to help [Republican senators] understand the connections and to reach the right decisions."

Regardless of the length of the trial and number of witnesses, Democrats are expected to rely on social media posts and videos taken during the storming of the Capitol, along with rioters' own words about their motivations for the attack, from the already extensive record of law enforcement charging documents.

GOP could challenge grounds for trial

While Republicans have criticized Trump for inciting the Capitol riot, only a few GOP senators have suggested a willingness to consider convicting the president. Pointing to the end of his term this week, some senators have argued that another trial would inflame tensions at the start of the Biden administration.

Sen. Lindsey Graham (R-S.C.), who led the Judiciary Committee in the last session of Congress, argued that Trump cannot be put on trial and convicted, since he will no longer be in office when the trial begins.

"If we embrace an unconstitutional impeachment of Donald Trump after he's out of office, it will destroy the party," Graham said on Fox News on Sunday.

That argument, supported by some conservative scholars, has met resistance from other legal experts who point to the precedent set by the Senate in holding a trial for Secretary of War William Belknap, who resigned his post days ahead of a House vote in 1876 in an unsuccessful attempt to avoid impeachment.

"When and if the former president goes to court to challenge his impeachment trial as unconstitutional, Congress is sure to make its argument based on these congressional precedents, as well as others, a case that would almost certainly make its way to the Supreme Court," J. Michael Luttig, a conservative former federal appeals court judge, wrote in the Washington Post.

Questions about Trump's involvement

With just days until the trial kicks off, Trump appears to lack any comprehensive legal strategy and has yet to organize a defense team, sources close to him say. The top lawyers who fiercely defended the president when he was impeached the first time have refused to represent him, say sources.

Trump's personal attorney Rudy Giuliani -- who sources say Trump had been considering to lead his defense -- told ABC News on Sunday that he won't be participating because his involvement as a speaker at the Jan. 6 rally makes him a potential witness.

Giuliani had initially told ABC News that he was working on the president's defense and that he was prepared to argue that Trump's claims of widespread voter fraud did not constitute incitement of violence because the widely-debunked claims are true.

Some Democrats worry that Trump simply won't take part in the proceedings and that he'll adopt a similar posture to his administration's broad rejection of congressional oversight and subpoenas during his time in office.

"I hope he is competently defended," Lawrence Tribe, a Harvard University Law School professor who has advised Democrats on their efforts to impeach Trump, told ABC News. "Otherwise part of what he'll be able to say in claiming victimization is that he was made a pariah ... therefore the verdict was illegitimate -- just as the election wasn't legitimate."

"I don't think it helps our history for him to be able to elaborate on that martyr story," Tribe said.

Since the House voted to impeach him, Trump has been asking top aides how a Senate trial could look. He even raised the idea of testifying himself -- but aides believe they have dissuaded him from pursuing such an option, sources said.