In the Democratic-led House, Speaker Nancy Pelosi will preside over the debate, a senior Democratic aide said
Donald Trump's flailing effort to overturn President-elect Joe Biden's victory comes down to the US Congress on Wednesday in a showdown led by a band of Republican lawmakers that could stretch proceedings past midnight but is almost certain to fail.
Although Biden won the November 3 election by more than 7 million popular votes and a 306-232 margin in the Electoral College, Trump -- without evidence -- continues to claim his victory was the result of widespread fraud.
State and federal reviews have knocked down allegations of significant fraud and legal efforts by Trump's allies to overturn the election have failed in multiple courts.
Biden is due to be sworn in as president on January 20.
Nevertheless, Senator Ted Cruz is expected to lead at least 11 other Republican senators, alongside dozens of Republicans in the House of Representatives, in objecting to Electoral College results being formally approved by both chambers on Wednesday.
The proceedings, the final step in the months-long process to select the US president, are typically brief and ceremonial. This year, it could drag into Thursday.
Cruz is bucking powerful Senate Majority Leader Mitch McConnell, who has recognized Biden's victory and urged his fellow Republicans not to pursue the challenges, which appear to lack the political support they would need to succeed.
Republican Senators including Josh Hawley and James Lankford have joined forces with Cruz, while other prominent members of the party, including Senator Mitt Romney and Representative Liz Cheney, oppose it.
The acrimony within the Capitol could pale in comparison to the protests outside, however. Trump has urged supporters, who include the violent Proud Boys, to take to the streets.
Trump will speak to protesters on the Ellipse behind the White House, according to a person briefed on his plans, and on Twitter has said the demonstrations "will be wild."
November and December protests involving the Proud Boys erupted in stabbings and brawls. Police have arrested the group's leader on charges of destruction of property related to an earlier protest and possession of a firearms magazine.
Many Republican senators who have refused to challenge the Nov. 3 results have received death threats on their office voice mail, which are being investigated by law enforcement officials, a senior Senate Republican aide said.
STATE BY STATE
The Electoral College results will be presented alphabetically, starting with Alabama. Republicans are expected to challenge results in the election battleground states of Arizona, Michigan, Wisconsin, Georgia, Pennsylvania and Nevada.
If the challenges are ultimately defeated, as anticipated, Vice President Mike Pence, acting in his role as president of the Senate, is expected to proclaim Biden and Senator Kamala Harris the next president and vice president.
The US Constitution does not give Pence the power to unilaterally overturn the results of the election but he is under pressure to do so from Trump, who delivered a not-so-subtle message on Twitter on Tuesday.
"The Vice President has the power to reject fraudulently chosen electors," Trump tweeted.
If at least one House member and one Senate member object to a state result, each chamber will hold separate debates for each state lasting up to two hours. Each would then vote to accept or reject the challenge and then report the result to the joint session of Congress, before moving onto the next challenge.
In the Democratic-led House, Speaker Nancy Pelosi will preside over the debate, a senior Democratic aide said. While Pence is due to preside over the House and Senate in joint session, he might hand off control of the Senate debate to senior Republican Senator Chuck Grassley.
House Majority Leader Steny Hoyer told reporters on Tuesday that Republican challenges will be rebutted by Democratic lawmakers from states being challenged.
In registering his objection, Cruz is expected to call for creating an emergency election commission to look into alleged voting irregularities, according to a source familiar with the upcoming deliberations.
The Republican maneuvering has created fissures within the Republican Party and among outside groups normally supportive of its efforts in Congress.
"After two months of recounts and legal challenges, not a single state recount changed a result and, of the dozens of lawsuits filed, not one found evidence of fraud or irregularities widespread enough to change the result of the election," Republican Senator Rob Portman said in a statement on Monday.
A group of seven House Republicans on Sunday pointed out that challenging the Electoral College system for US elections could weaken public support for the very system that improves their party's chances of winning the presidency.
"From a purely partisan perspective, Republican presidential candidates have won the national popular vote only once in the last 32 years," the group said in a statement. "They have therefore depended on the Electoral College for nearly all presidential victories in the last generation."
Business groups have also called for quick certification of the tallies. Chamber of Commerce CEO Thomas Donohue said any effort to disregard the results "undermines our democracy and the rule of law."