The GOP campaign chief hopes to tone down his opposition after prodding after the midterm showdown.
At a closed-door party luncheon Tuesday afternoon, Sen. Rick Scott announced a challenge to McConnell.
It was the first such protest for the Senate GOP leader in 15 years.
Scott said, "Not satisfied with the status quo, he will not lose to the senator from Florida for the first time."
When GOP senators gather for leadership election on Wednesday, the contest will heat up.
Initially, Republicans are expected to delay the election until the Georgia and Alaska Senate races are determined.
McConnell is expected to win a two-year term if the effort to delay the election fails.
McConnell said, "I have the votes. I will get elected. The only issue is whether it will happen sooner or later."