Robert O’Brien, former President Trump’s final national security adviser, announced his endorsement of J.D. Vance’s Ohio Senate campaign, offering the Buckeye State Republican a boost in a crowded primary field.

O’Brien in a statement released by Vance’s campaign cast the “Hillbilly Elegy” author and venture capitalist as a “strong leader who will fight for President Trump’s America First foreign policy.”

“From dismantling the Big Tech alliance that is increasingly selling out to the Chinese Communist Party, to bringing back manufacturing jobs to America’s working class, JD will fight to reverse American decline. Conservatives and Trump supporters across Ohio have a champion in JD Vance, and I believe he is the best choice to serve as the next United States Senator from the Buckeye State,” O’Brien said.

The endorsement is a boon for Vance, who burst onto the scene in 2016 with his book detailing his upbringing in Appalachian Ohio and he touted as insights into the voters who still live there. He is one of five Republicans running in the primary for the open seat being vacated by Sen. Rob Portman (R). 

The primary has largely revolved around the five contenders’ support for Trump, who is looming large in the Buckeye State and Senate races across the country.

The support from O’Brien could bolster Vance’s pro-Trump bona fides. Kellyanne Conway, a senior counselor in the Trump administration, is advising businessman Bernie Moreno, and Vance has had to distance himself from past comments criticizing Trump. 

O'Brien served as Trump's fourth national security adviser from September 2019 until the end of the administration in January.

“Ambassador O’Brien is one of the country’s most influential and well-respected leaders. As our country and the world descend into chaos, I’m honored to receive his endorsement. Robert is one of the great champions of President Trump’s America First policy agenda, and his endorsement sends a strong signal about who will stand up to our enemies,” Vance said.

Other Republicans running in the primary include former state Treasurer Josh Mandel; Jane Timken, the former head of the Ohio Republican Party; and businessman Mike Gibbons.

Trump has yet to issue an endorsement but held a meeting with the candidates earlier this year in Florida which resembled an audition for his support.

While Vance will have to balance his support for Trump with his past criticism, he does enter the race with a number of distinct advantages, including broad name recognition from his book and the support from megadonor Peter Thiel, who gave $10 million to a super PAC backing Vance.

Rep. Tim Ryan and progressive activist Morgan Harper are running in the Democratic Party.

Ohio, once a key battleground, has tilted to the right in recent years, with Trump winning it by about 8 points in both 2016 and 2020.