Sen. Rand Paul is backing state Sen. Kelli Ward in the GOP primary in the newly open Arizona Senate race. The Kentucky Republican is the first sitting senator to back Ward’s campaign.

Ward was originally challenging GOP Sen. Jeff Flake, who has been sharply critical of President Donald Trump. But with Flake recently announcing his retirement, the field is now open for other GOP hopefuls to run for Senate. Ward has also received support from former Trump adviser Steve Bannon, who has said he wants to take on every GOP Senate incumbent except Sen. Ted Cruz, R-Texas.

“Honored to have the support of Senator @RandPaul — AZ needs a US Senator who values #liberty, freedom, & the #Constitution #NewGOP” Ward’s campaign account tweeted Wednesday. 

Paul cited their similar experiences as doctors — Ward in family medicine and Paul as an eye doctor — in his statement endorsing Ward.

“As a fellow physician, Dr. Ward witnessed firsthand the damage Obamacare has inflicted on our nation’s healthcare system,” said Paul. “As senator, she will bring a new voice and a fresh perspective to the Senate.”

Ward challenged Sen. John McCain in the Republican primary last year, and was soundly defeated. She struggled with fundraising and did not receive the backing of some major outside groups.

The GOP primary could be divisive depending on who jumps in the race. GOP Rep. Martha McSally, who represents swing district in Tucson, has reportedly expressed interest in the race. But a coalition of conservative groups including the Club for Growth PAC, Senate Conservatives Fund, and FreedomWorks said McSally is a liberal lawmaker.

“If elected to the Senate, she will undermine the conservative agenda to expand freedom and limit the size of government,” said FreedomWorks president Adam Brandon.

The Club for Growth PAC separately called on former GOP Rep. Matt Salmon to run for Senate in Arizona.

Democrats seized on Paul’s endorsement to highlight the Republican divide.

“Rand Paul’s endorsement of Kelli Ward piles onto the chaotic nightmare within the Arizona Republican Party’s civil war,” said Arizona Democratic Party spokesman Drew Anderson. “Ward represents the extreme partisan discord that has Arizonans fed up with Washington, and she has no place in the United States Senate.”

McSally and the other members of Republican House members from Arizona huddled last week to discus the race.

Rep. Trent Franks, who does not plan on running for Senate, said the delegation has “an unusual closeness.”

“We had a long discussion with the foundational objective being to do what’s right for the country,” Franks said after the meeting. “I believe that we were able to be transparent with each other and that alone was valuable.”

GOP Rep. David Schweikert has also ruled out a run. But a slew of other contenders could jump in the race. Jay Heiler, a member of the Arizona Board of Regents, has also formed an exploratory committee.

Eric Garcia contributed to this report.