(CNN) - Republican Sen. Lamar Alexander of Tennessee, who's up for a third term next year, now has a GOP primary challenger.
State Rep. Joe Carr announced Tuesday he will run for the U.S. Senate, taking on the state's senior senator in 2014.
"This is a David and Goliath matchup," Carr told CNN. "We're going against a well-established and well-funded candidate...But we liked the way the story of David and Goliath ends, and so we're very encouraged."
Carr was originally planning to challenge Republican Rep. Scott DesJarlais in the state's 4th U.S. Congressional District, but dropped his bid in order to take on Alexander.
His campaign manager, a well-known Republican operative Chip Saltsman, resigned from Carr's congressional campaign upon learning of the state representative's change in plans.
In his letter of resignation to Carr, Saltsman said in part, "Having learned of your decision to run for the United States Senate, I must resign from your campaign. I signed up to help you run for Congress, not the Senate."
Saltsman, a former Tennessee GOP chairman, said he's long been a supporter of Alexander and will continue to stand by the senator, who has backed many Republicans in the state seeking office on the local, state and federal level.
"It is because of Lamar Alexander that people like you have the honor of serving in the majority of the state legislature," Saltsman wrote in the letter.
Carr said Saltsman's letter "was expected," as he informed Saltsman of his Senate decision on Monday. Carr said their conversation was "amicable," and Saltsman agreed with that assessment in an email to CNN.
As for why Carr decided to abandon his House bid, the state representative said that as he began raising money in the district and meeting with outside conservative groups in Washington, D.C., he said he was frequently asked to consider challenging Alexander.
With the "intensity" of those questions increasing in the past few weeks, Carr said he made the decision to launch a Senate bid.
"After a serious amount of prayer and thought, we have determined this is a course of action that we need to take," he said.
He declined to identify the groups that he met with, but hopes that he can announce their endorsements "at the appropriate time."
As Carr has acknowledged, his challenge will likely be an uphill battle. Alexander handily won his last election with 65% of the vote and has 10 times the amount of campaign cash. And the incumbent has already gone up with ads this summer for his 2014 bid.
"Sen. Alexander has a conservative voting record," a spokesman for Alexander's campaign said, responding to Carr's announcement. "He has an A rating from the NRA and a 100 percent rating from both National Right to Life and the U.S. Chamber of Commerce. He will continue to do his best to use his experience and conservative principles to solve problems and get results for the people of Tennessee."
Carr, however, argued Alexander is out of step with Tennessee voters, saying the state needs someone who will push hard for defunding Obamacare and stand up against the Senate immigration bill, or as he called it the "amnesty bill," that passed this summer. Alexander was among 14 GOP senators who voted for the legislation, which has so far gone nowhere in the House.
Carr said he raised well over $300,000 during his congressional campaign this summer, an amount that pales in comparison to Alexander's healthy war chest of $3.1 million.
Following his public announcement on the Nashville radio station, WTN, the state representative quickly had the support of the group Tea Party Nation.
"Tennessee conservatives should unite around Joe Carr, a patriot and a great candidate," stated an email from Judson Phillips, founder of Tea Party Nation.
Meanwhile, the Senate Conservatives Fund, an independent group that backs conservatives in GOP primary bids, has not given its backing to Carr.
"We're a little concerned about Carr. If he couldn't get traction in the House race, he probably won't get traction in the Senate race either," said Matt Hoskins, executive director of the group. "If Carr doesn't catch fire and a more compelling conservative enters the race, he could be the spoiler that helps re-elect Lamar Alexander. That's something we want to avoid."
The Club for Growth, another DC-based group that backs conservative candidates, says that it's watching the race. A source close to the group tells CNN that it met with Carr when the state lawmaker was considering a House bid.
Carr could run into some hot water over a report last year that he defended Republican Rep. Todd Akin, who made the controversial argument last year that women are biologically capable of preventing pregnancies in cases of "legitimate rape." While the congressman, who was running for a U.S. Senate seat in Missouri at the time, apologized for the remark, the comment spurred strong denouncement from GOP leaders who urged Akin to withdraw his candidacy. Akin stayed in the race but ultimately lost.
During the Republican National Convention in August–just weeks after Akin made the comment–Carr reportedly stood by Akin, taking a position that Akin should not drop out of the race. He also argued there was likely a scientific basis to Akin's remark, according to the Memphis Flyer. Carr, however, did not agree with the use of the term "legitimate rape."
Carr told the Tennessean back in August he was misquoted in the Memphis Flyer about the scientific argument, but maintained Akin should stay in the race.
- CNN Political Editor Paul Steinhauser and CNN Political Reporter Peter Hamby contributed to this report.