Tennessee state Rep. Joe Carr (R) announced Tuesday that he will challenge Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderWeek ahead: Lawmakers near deal on children's health funding Ryan suggests room for bipartisanship on ObamaCare Time to end fiscal year foolishness MORE (Tenn.) in the Republican primary.

On local radio, Carr said he was running because Alexander was "the most liberal" member of the state's GOP delegation. 

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Carr had been focused on running for the House in 2014 against embattled Rep. Scott DesJarlais (R-Tenn.) before switching his focus to Alexander.

Carr said that while he was in a "genuine position" to defeat DesJarlais, many supporters had urged him to challenge Alexander instead.

Conservatives have long warned of a coming challenge to Alexander, but Carr is the first to enter the race. 

A coalition of 20 Tennessee Tea Party and conservative groups published an open letter last week urging Alexander to “retire with dignity” rather than seek a third term.

"Unfortunately, our great nation can no longer afford compromise and bipartisanship, two traits for which you have become famous,” the letter read.

The coalition criticized Alexander for his willingness to work with "those who are actively undermining [conservative] values."

Alexander responded to the letter in a Tuesday editorial in the Tennessean defending his record in the Senate.

"I appreciate the suggestion, but if the people of Tennessee will allow it, I’d rather continue to serve — hopefully, with dignity," he wrote.

In the op-ed, Alexander touts his work as governor of the state, former Arkansas Gov. Mike Huckabee's (R ) support and his efforts to protect fishing rights and keep student loan rates low, among others.

Alexander is only the latest prominent GOP senator to face a conservative primary challenge. Senate Minority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellSessions: 'We should be like Canada' in how we take in immigrants NSA spying program overcomes key Senate hurdle Overnight Finance: Lawmakers see shutdown odds rising | Trump calls for looser rules for bank loans | Consumer bureau moves to revise payday lending rule | Trump warns China on trade deficit MORE (Ky.) and Sen. Lindsey GrahamLindsey Olin GrahamDHS chief takes heat over Trump furor Overnight Defense: GOP chair blames Dems for defense budget holdup | FDA, Pentagon to speed approval of battlefield drugs | Mattis calls North Korea situation 'sobering' Bipartisan group to introduce DACA bill in House MORE (S.C.) are also facing primary foes.

According to The Tennessean, other potential challengers to the Tennessee senator include Knox County Mayor Tim Burchett and former Williamson County Chairman Kevin Kookogey. Local activists will vet potential challengers next month.

Updated at 2 p.m.