WASHINGTON — President Trump’s extraordinary challenge of his election defeat by President-elect Joe Biden is becoming a defining moment for the Republican Party before next week’s joint session of Congress to confirm November’s electoral college results.

Senate Majority Leader Mitch McConnell of Kentucky is urging Republicans not to try to overturn the election, but not everyone is heeding him. Sen. Josh Hawley of Missouri vows to join House Republicans in objecting to the state tallies. On the other side of the party’s split, Sen. Ben Sasse of Nebraska warns such challenges are a “dangerous ploy” that threaten the nation’s civic norms.

Vice President Mike Pence faces growing pressure from Trump’s allies over his ceremonial role in presiding over the session Wednesday.

The days ahead are not expected to change the outcome. Biden is set to be inaugurated Jan. 20 after winning the electoral college vote, 306 to 232. But the effort to subvert the will of voters is forcing Republicans to make choices that will set the contours of the post-Trump era and an evolving GOP.

“I will not be participating in a project to overturn the election,” Sasse wrote in a lengthy social media post.

Sasse, a potential 2024 presidential contender, said he was “urging ... colleagues also to reject this dangerous ploy.”

Trump’s allegations of voter fraud have been consistently debunked by nonpartisan and GOP election officials, and courts — including the Supreme Court — have dismissed or dropped scores of lawsuits filed by Trump and his allies challenging the election results.

Still, the president has pushed Republican senators to pursue his unfounded charges, even though the electoral college has already cemented Biden’s victory and all that’s left is Congress’ formal recognition of the count before the Democrat is sworn in as president.

“We are letting people vote their conscience,” South Dakota Sen. John Thune, the second-ranking Republican, told reporters at the Capitol.

As the GOP whip, Thune would be in charge of rounding up votes if Republican leaders chose to put their muscle behind Trump’s demands. Instead, he noted the gravity of questioning the election outcome.

“This is an issue that’s incredibly consequential, incredibly rare historically and very precedent-setting,” Thune said. “This is a big vote. They are thinking about it.”

Pence will be carefully watched as he presides over what is typically a routine vote count in Congress but is now heading toward a prolonged showdown that could extend into Wednesday night, depending on how many challenges Hawley and others mount.

On Friday, in another blow to Trump’s efforts to remain in the White House, a federal judge dismissed a last-ditch lawsuit against Pence that aimed to give him the power to overturn the electoral college results.

McConnell convened a conference call with GOP senators Thursday to address the coming joint session and logistics of tallying the vote, according to several Republicans who were granted anonymity to discuss the private call.

The GOP leader called on Hawley to answer questions about his challenge to Biden’s victory, according to two of the Republicans. But there was no response because Hawley did not join the call, they said.

Hawley’s office did not respond to a request for comment.

Sen. Patrick J. Toomey (R-Pa.), who has acknowledged Biden’s victory and defended Pennsylvania’s elections systems as valid and accurate, spoke up on the phone call, objecting to those challenging his state’s results and making clear he disagrees with Hawley’s plan to contest the electoral college count, Toomey’s office said in a statement.

McConnell had previously warned GOP senators not to participate in raising objections, saying it would be an ill-advised vote for colleagues. In essence, lawmakers would be forced to choose between the will of the outgoing Republican president and that of the voters.

Several Republicans have indicated they are under pressure from their constituents to show they are fighting for Trump in his baseless campaign to stay in office.

Hawley became the first GOP senator this week to announce he would raise objections when Congress meets to affirm that Biden won, which would force House and Senate votes that are likely to delay — but in no way alter — the final certification of Biden’s victory.

Other Republican senators are expected to join Hawley, wary of ceding the spotlight to him as they, too, try to emerge as leaders in a post-Trump era.

A number of Republicans in the Democratic-majority House have already said they will object on Trump’s behalf. They only needed a single senator to go along with them to force votes in both chambers.

When Biden was vice president, he presided over the session when the electoral college presented the 2016 vote tally to confirm Trump had won. The session was brief, despite objections from some Democrats.

Jen Psaki, speaking for Biden’s transition team, dismissed Hawley’s move as “antics” that would have no bearing on the president-elect’s Jan. 20 inauguration.