Tennessee state Rep. Joe Carr (R) announced Tuesday that he will challenge Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderOvernight Health Care: House passes .3T omnibus | Bill boosts funds for NIH, opioid treatment | Senators spar over ObamaCare fix | 'Right to Try' bill heads to the Senate Overnight Regulation: Omnibus includes deal on tip-pooling rule | Groups sue over rules for organic livestock | AT&T, DOJ make opening arguments in merger trial Warren presses Mulvaney, Azar on tip pooling 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. 

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 McConnellYou just can't keep good health policy down Trump threatens to veto omnibus over lack of wall funding, DACA fix Democrats desperate for a win hail spending bill MORE (Ky.) and Sen. Lindsey GrahamLindsey Olin GrahamBernie Sanders to Trump: Firing Mueller 'an impeachable offense' The Memo: Lawyer’s exit signals harder line by Trump Senators introduced revised version of election cyber bill 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.