Trump presses, but Pence tells him he can't reject Biden's win before Congress

Vice President Mike Pence faces perhaps his greatest political challenge on Wednesday when he oversees the joint session of Congress at which he'll be required to announce Joe Biden as president-elect, forced to choose between his long-standing loyalty to the Constitution and a president demanding he do something the law doesn't allow.

"The Vice President has the power to reject fraudulently chosen electors," Trump tweeted Tuesday, keeping up the pressure on Penceeven though the vice president's role as laid out by the Constitution does not give him that power.

There are also no "fraudulently chosen electors"; each state has certified its results and sent electors to the Electoral College, which confirmed Biden's win last month.

On Wednesday morning, Trump tweeted that Pence needed to show "extreme courage."

It wasn't until just mid-day Tuesday that Pence and his office openly confirmed that he would even show up Wednesday -- despite previously privately indicating he would -- when his chief of staff, Marc Short, told ABC News that Pence did, indeed, plan to preside.

At one point during the day Tuesday, Pence told Trump he does not have the power to reverse the outcome of the November election when he presides over the joint session, two sources familiar with the matter confirmed to ABC News, in a development first reported by the New York Times.

Trump on Monday had made clear to Pence privately that he expected him to use his role as president of the Senate to deny Biden the presidency during the joint session, in which both houses will count each state's electoral votes and reaffirm Biden's win, according to a person familiar with their conversation.

Trump met with Pence in the Oval Office before flying to Georgia for a campaign rally where he publicly pressured the vice president. They met in the Oval Office again on Tuesday, according to a person familiar with the meeting.

“I hope Mike Pence comes through for us, I have to tell you," Trump said Monday night in Georgia. "I hope that our great vice president -- our great vice president comes through for us. He’s a great guy. Of course, if he doesn't come through, I won't like him quite as much."

The vice president's role as laid out by the Constitution and a 19th-century law is ceremonial, though, and Pence has no real power beyond reading a script, opening envelopes that show states' electoral votes and announcing Biden as the winner.

Pence has spent a significant amount of time meeting with the Senate parliamentarian, and those close to him say he’s unlikely to divert from that scripted role.

Trump puts Pence on spot with fringe legal theory

The president's Republican allies on Capitol Hill -- 13 senators and more than one hundred House members -- plan to object to certain swing states' votes after Trump has made baseless claims of voting fraud there, in what would, in effect, be an attempt to overturn the election results, although some claim their real concern is "election integrity."

The effort is all but certain to fail.

Under federal law, a member of the House or Senate can contest the Electoral College results from any state, forcing the House and Senate to separate for up to two hours of debate and vote on whether to accept a slate of electors. A majority of both chambers would have to support the motion to successfully challenge a given slate of electors.

The president has been pushing them to do so, even though doing so will only prolong the session -- rather than change the ultimate result -- since the Democrat-controlled House and enough Senate Republicans are expected to join Senate Democrats to reject any objections.

In recent days, he has subscribed to a fringe theory promoted by John Eastman, a law professor who had represented Trumpin a failed quest to have the Supreme Court intervene in the election, that Pence would, in fact, be able to make a difference. Eastman was part of Trump's Monday Oval Office discussion with Pence, according to the person familiar with their conversation.

Pence balancing loyalty to Trump with devotion to Constitution

Pence is seeking to balance his political future -- which he has staked on his loyalty to Trump -- on his commitment to the Constitution.

How Trump reacts to Pence's handling of the congressional session could define the vice president's political future -- and any hope of success if he runs for president down the line.

Pence has long viewed himself as a conservative who respects the view of the document's framers, as well as lawmakers' original intent.

"I was a teenager when I first began to study and understand the Constitution of the United States, and it has been a decades-long love affair with this document," Pence told newly naturalized American citizens at a ceremony in Washington last year.

To prepare, Pence has read and studied legal opinions on the Electoral Count Act, an 1887 law that further proscribes Wednesday's session -- particularly when objections are raised, in addition to consulting those inside and outside his office, according to a Trump administration official. On Sunday, he met with Senate parliamentarian in his office at the Capitol, the official said.

"He's done his due diligence in order to better inform himself of what his role is," the official said.

A close friend of Pence’s, David McIntosh, told ABC News that the vice president would likely be guided by his “really deep, residing respect for the separation of powers.”

“He will consciously do as much as he can, I think, but be constrained by what he sees as the constitutional restraints and authority that he has as president of the Senate,” McIntosh, the president of the conservative group Club for Growth, said.

Pence has also reviewed how his predecessors handled the congressional vote count in the most recent few instances, including Vice President Al Gore graciously presiding over his loss in 2001 following a contentious election, the official said.

The vice president has walked a fine line in the two months since he and Trump lost reelection, indulging Trump's baseless accusations of fraud while staying away from the president's wilder conspiracy theories.

He has been less than full-throated in his support of Trump's pursuit to overturn the election results by pushing conspiracy theories about electoral fraud, but his office did put out a statement saying he "welcomes" the challenge at the Wednesday's joint session.

“Vice President Pence shares the concerns of millions of Americans about voter fraud and irregularities in the last election,” his chief of staff Short said in a statement on Saturday. “The Vice President welcomes the efforts of members of the House and Senate to use the authority they have under the law to raise objections and bring forward evidence before the Congress and the American people on January 6th.”

There is no evidence of widespread voter fraud or irregularities that would alter the November election’s outcome.

Still, he has expressed support for Trump's legal fights -- the president and his allies have lost 50 cases challenging the results -- and on Monday, he empathized with Trump's supporters during a campaign event in Georgia.

"We all got our doubts about the last election," Pence said. "And I want to assure you, I share the concerns of millions of Americans about voting irregularities."