Republicans from the US House and Senate have objected to the counting of Arizona's electoral vote, forcing votes in both chambers on Joe Biden's victory in the state.
The objection was made by Arizona Representative Paul Gosar and was signed by Texas Senator Ted Cruz, both Republicans.
The two chambers now have two hours to debate the challenge.
Earlier, a band of US President Donald Trump's Republican allies said they would challenge his election loss to Biden in a marathon joint session of the US Congress in a showdown that is almost certain to fail.
The Republican-led Senate and Democratic-controlled House of Representatives were on Wednesday meeting to formally certify Biden's victory in the November 3 election in proceedings that could stretch past midnight.
In a joint session of Congress, Trump's allies plan to challenge the results from a handful of states won by Biden.
Thousands of pro-Trump protesters converged on Washington DC ahead of the session.
Biden won the election by a 306-232 count in the state-by-state Electoral College and by a margin of more than 7 million ballots in the popular vote but the Republican president continues to claim there was widespread fraud and that he was the victor.
Cruz, seen as a potential 2024 presidential candidate, is expected on Wednesday afternoon to lead at least 11 other Republican senators, alongside a majority of the 211 Republicans in the House of Representatives, in objecting to Electoral College results being formally approved by the two chambers of Congress.
The proceedings began at 1pm for Congress to formally certify the presidential election results, the final step in a months-long process, normally would be ceremonial and perfunctory.
This year, the proceedings could drag into Thursday.
Vice President Mike Pence will be closely watched as he presides over the session.
Pence has a largely ceremonial role, opening the sealed envelopes from the states after they are carried in mahogany boxes used for the occasion and reading the results aloud.
While other vice presidents, including Al Gore and Richard Nixon, also presided over their own defeats, Pence supports those Republican lawmakers mounting challenges to the 2020 outcome.
It's not the first time lawmakers have challenged results, with the Democrats doing so in 2017 and 2005, but the intensity of Trump's challenge is like nothing in modern times and an outpouring of current and elected Republican officials warn the showdown is sowing distrust in government.
The Electoral College results will be presented alphabetically, starting with Alabama.
Republicans are expected to challenge results in Arizona, Michigan, Wisconsin, Georgia, Pennsylvania and Nevada.
If the challenges are ultimately defeated as anticipated, Pence, acting in his role as president of the Senate, is expected to proclaim Biden the next president and Senator Kamala Harris as the next vice president.
If at least one House member and one Senate member object to a state result, each chamber would hold separate debates for each of those states lasting up to two hours.
Each chamber 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 registering his objection, Cruz is expected to call for the creation of an emergency election commission to look into voting irregularities claimed by Trump's allies, a source familiar with the deliberations said.