WASHINGTON — Senate Minority Leader Mitch McConnell is facing a brewing challenge to his position as the top Republican in the Senate.

Several senators are campaigning to delay a vote scheduled for next week that was expected to return McConnell to the top job.

The Kentucky Republican has held the position since 2007, making him the longest-serving GOP leader in Senate history.

Going into the midterm elections Tuesday, McConnell appeared to have a firm grasp on the leader’s job and Republicans appeared poised to take control of the Senate. But now, however, both remain uncertain.

McConnell has long been the subject of ire from former President Donald Trump, who has campaigned aggressively to have him ousted as leader. McConnell publicly blamed Trump for the Jan. 6 riot.

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No Republican senator has announced their intention to run against McConnell. But Florida Sen. Rick Scott’s name has been floated repeatedly, including by Trump.

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

Those suggesting a delay have pointed to the remaining unknowns. NBC News has not yet projected winners in Arizona or Nevada, which will be pivotal in deciding control of the Senate. And the Georgia Senate contest now comes down to a Dec. 6 runoff, which could decide control if Republicans don’t win both Nevada and Arizona.

Three Republican senators — Scott; Mike Lee of Utah, and Ron Johnson of Wisconsin — circulated a letter this week calling for the delay, saying Senate Republicans need more time to vet possible candidates. The letter was first reported by Politico and a copy was obtained by NBC News.

“We should cast our votes only after being fully informed of where each candidate stands on Conference priorities for the coming Congress,” the letter states.

Asked in January what his agenda would be if Republicans took control of the Senate, McConnell deflected.

“That is a very good question,” McConnell said. “And I’ll let you know when we take it back.”

The closed-door party elections, which also install other leadership positions like the whip, are scheduled to take place on Wednesday.

Sen. Marco Rubio, R-Fla., also called for a delay, writing on Twitter that “we need to make sure that those who want to lead us are genuinely committed to fighting for the priorities & values of the working Americans (of every background) who gave us big wins in states like #Florida.”

Republican Sens. Josh Hawley of Missouri and Cynthia Lummis of Wyoming echoed Rubio on Twitter.

“Exactly right. I don’t know why Senate GOP would hold a leadership vote for the next Congress before this election is finished,” Hawley wrote. “We have a runoff in #GASenate — are they saying that doesn’t matter? Don’t disenfranchise @HerschelWalker.” 

A Rubio adviser said the senator is concerned McConnell has not laid out a vision for the future. 

“We are suddenly finding ourselves very likely in the minority. We just lost a whole bunch of winnable races,” the adviser said. “Let’s figure out how we’re going to spend the next two years fighting for this sort of multiracial working-class party that we want to build.”

The adviser, who was granted anonymity to speak candidly, refused to say if Rubio himself would consider a run for leader.

“What he’s saying is, it doesn’t make sense to have this election right now while we have some of these fundamental questions we need to ask ourselves,” the adviser said.

In the letter from Lee, Johnson and Scott, the senators were also critical of the way McConnell has been running the caucus.

“The Senate Republican Conference must change the way it operates — regardless of the outcome of the still-pending elections,” the letter states. “We must be vocal, sell our ideas every day to the American public, constantly articulating what we plan to accomplish to help American families, and what we are doing to put our plan into action.”