The plan to object to Biden's electors has added importance to a usually routine step in the path to inauguration and is the broadest effort to challenge a president's win since the Civil War.
The Republicans - at least 13 senators and potentially more than 100 House members - are citing Trump's repeated, baseless charges of widespread fraud. Their objections could force multiple votes in the Republican-run Senate and the Democratic-controlled House that will almost certainly fail. More than a dozen GOP senators have said they won't support the challenges and plan to vote against them.
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There was not widespread fraud in the election, as has been confirmed by a range of election officials and by William Barr, who stepped down as attorney general last month. Neither Trump nor any of the lawmakers promising to object to the count have presented credible evidence that would change the outcome.
Nearly all of the legal challenges put forth by Trump and his allies have been dismissed by judges. The Supreme Court, which includes three Trump-nominated justices, has also denied requests to hear a pair of cases aimed at invalidating the outcome of the election in key battleground states.
The congressional meeting on Wednesday is the final step in reaffirming Biden's win, after the Electoral College officially elected him in December. The meeting is required by the Constitution and includes several distinct steps.
A look at the joint session:
What happens when Congress meets Wednesday?
Under federal law, Congress must meet Jan. 6 to open sealed certificates from each state that contain a record of their electoral votes. The votes are brought into the chamber in special mahogany boxes used for the occasion.
Representatives of both parties in both chambers read the results out loud and do an official count. The president of the Senate, Vice President Mike Pence, presides over the session and declares the winner. The session begins at 1 p.m. EST.
What does the Constitution require?
The Constitution requires Congress to meet and count the electoral votes. If there is a tie, then the House decides the presidency, with each congressional delegation having one vote. That hasn't happened since the 1800s, and Biden's electoral win over Trump was decisive, 306-232.
How does the session unfold?
The two chambers meet together midday to count the votes. Pence is expected to preside, but if the vice president cannot be there, there is precedent for the Senate pro tempore, or the longest-serving senator in the majority party, to lead the session. That's currently Sen. Chuck Grassley, R-Iowa.
The presiding officer opens and presents the certificates of the electoral votes in alphabetical order of the states. The appointed "tellers" from the House and Senate, members of both parties, then read each certificate out loud and record and count the votes. At the end, the presiding officer announces who has won the majority of votes for both president and vice president.
What if there's an objection?
After a teller reads the certificate from a state, any member can stand up and object to that state's vote on any grounds. But the presiding officer will not hear the objection unless it is in writing and signed by both a member of the House and a member of the Senate.
If there is such a request, then the joint session suspends and the House and Senate go into separate sessions to consider it for up to two hours. For the objection to be sustained, both chambers must agree to it by a simple majority vote. If they do not both agree, the original electoral votes are counted with no changes.
This process repeats each time there is an objection, and could go all night and into Thursday.
The last time such an objection was considered was 2005, when Rep. Stephanie Tubbs Jones of Ohio and Sen. Barbara Boxer of California, both Democrats, objected to Ohio's electoral votes, claiming there were voting irregularities. Both the House and Senate debated the objection and easily rejected it. It was only the second time such a vote had occurred.
Who is expected to object?
Dozens of House Republicans and a smaller group of GOP senators are expected to object to the count from at least six swing states where Trump has alleged fraud, despite the consensus of nonpartisan election officials and even Trump's former attorney general that there was none. None of the members has presented detailed evidence and few of them have called into question the elections of congressional lawmakers who won election on the same ballots.
The House Republicans, led by Ohio Rep. Jim Jordan and Alabama Rep. Mo Brooks, have said they plan to object to the results from Arizona, Georgia, Michigan, Nevada, Pennsylvania and Wisconsin.
In the Senate, Missouri Sen. Josh Hawley was the first to say he would join with the House Republicans to object to the Pennsylvania results. On Saturday, Sen. Ted Cruz of Texas announced a coalition of 11 additional senators who vowed to vote against state electors on Wednesday unless Congress appoints an electoral commission to immediately conduct an audit of the election results. Hawley and Cruz are both among potential 2024 presidential contenders.
Sen. Kelly Loeffler, R-Ga., said at a rally with Trump on Monday that she will also join in on the challenges.
It is unclear if senators will join with the House Republicans on all six states. Cruz is expected to object to Arizona, which comes early in the count, and Loeffler may object to her home state of Georgia, according to people familiar with the planning who requested anonymity to discuss it.
The challenges have split the party. Senate Republican leader Mitch McConnell has urged his colleagues not to object, saying last month on a private call that the vote would be "terrible."
Several other Senate Republicans have criticized the effort as well, including Texas Sen. John Cornyn and South Dakota Sen. John Thune, the No. 2 Senate Republican. Thune said last month that any objections will go down "like a shot dog" in the Senate.
Nebraska Sen. Ben Sasse said the challenge is "bad for the country and bad for the party." South Carolina Sen. Tim Scott said that "no justice, judge or fact finder has found evidence indicating the election results in those states should be overturned."
Arkansas Sen. Tom Cotton, another possible contender in 2024, said challenging the results "would take away the power to choose the president from the people, which would essentially end presidential elections and place that power in the hands of whichever party controls Congress."
What is Pence's role?
Pence's role is largely ceremonial and he has no power to affect the outcome, despite Trump's wishes to the contrary.
The role of the vice president as presiding officer is often an awkward one, as it will be for Pence, who will be charged with announcing Biden's victory - and his own defeat - once the electoral votes are counted.
Other vice presidents have been in that uncomfortable situation. In 2001, Vice President Al Gore presided over the counting of the 2000 presidential election he narrowly lost to Republican George W. Bush. Gore had to gavel several Democrats' objections out of order. In 2017, Biden presided over the count that declared Trump the winner. Biden also shot down objections from House Democrats that did not have any Senate support.
Once Congress counts the votes, what's next?
The joint session is the last official chance for objections, beyond court cases that have so far proven ineffective for Trump and his team.
"I think there comes a time when you have to realize that despite your best efforts you've been unsuccessful," Cornyn said last month.
AP Congressional Correspondent Lisa Mascaro contributed to this report.