EXCLUSIVE: A key ally of Senate Minority Leader Mitch McConnell, R-Ky., has decided which candidate he will support in one crucial swing state race that could ultimately determine whether Republicans win back control of the Senate in 2024.

Sen. Tom Cotton, R-Ark., announced Monday his endorsement of Republican Arizona Senate candidate Kari Lake, a conservative firebrand and former 2022 gubernatorial candidate, in her bid to flip the seat currently held by independent Sen. Kyrsten Sinema, a former Democrat.

"Kari Lake is the fighter Arizona needs to secure the border, unleash American Energy Independence, and lower the cost of living. Joe Biden’s policies have been a disaster for Arizona and Kari Lake will stand up to the Biden agenda and put Arizonans first. Let’s send Kari to the Senate and take back the majority," Cotton said in a statement to Fox News Digital.

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Republican Arkansas Rep. Tom Cotton

Sen. Tom Cotton says GOP Senate candidate Kari Lake is the "fighter Arizona needs to secure the border" and "lower the cost of living." (Tom Williams/CQ-Roll Call Inc. via Getty Images)

"I am grateful for the endorsement of Sen. Cotton, someone who served our country honorably in the military and has been a conservative warrior in the Senate. I look forward to working with Sen. Cotton to secure our border and get America back on track from Joe Biden and Chuck Schumer's far-left agenda," Lake told Fox.

The race is expected to be one of the most closely watched of the 2024 election cycle as Republicans aim for a majority in Congress' upper chamber, which Democrats currently control by a slim 51-49 margin with the support of Sinema and her fellow independents, Bernie Sanders of Vermont and Angus King of Maine.

With Cotton's backing, Lake has secured the support of two top senators, the other being Sen. John Barrasso, R-Wyo., as well as House GOP Conference Chair Elise Stefanik of New York and a number of other House Republicans. She also has the backing of former President Trump, who endorsed her via video during her campaign launch in October, and multiple state and local officials in Arizona and across the country.

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Republican Arizona Senate candidate Kari Lake

Former Arizona gubernatorial candidate Kari Lake arrives to autograph books ahead of a rally with former President Trump at Ted Hendricks Stadium at Henry Milander Park on Nov. 8, 2023, in Hialeah, Florida. (Jabin Botsford/The Washington Post via Getty Images)

Lake's only major challenger in the race for the Republican nomination so far is Pinal County Sheriff Mark Lamb.

Whichever Republican wins the nomination will likely face Phoenix-area Democrat Rep. Ruben Gallego in the general election.

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Sinema has not yet said whether she will run for reelection.

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.