FreedomWorks PAC Executive Director Max Pappas said the PAC had been overwhelmed by hundreds of emails and phone calls from supporters across Ohio urging the national group to support Mandel.

“Josh Mandel has a real pro-growth agenda that will release Ohio’s economy from the shackles of government overregulation, and allow businesses to start creating jobs again," Pappas said.

Mandel's challenge to Democratic Sen. Sherrod BrownSherrod Campbell BrownSenate campaign fundraising reports roll in Commerce sends Trump long-awaited steel report GOP Rep. Jim Renacci announces Ohio Senate bid MORE (Ohio) has attracted the interest and support of other big-league fiscally conservative groups, including the Club for Growth and Sen. Jim DeMint's (R-S.C.) Senate Conservatives Fund. Sen. Marco RubioMarco Antonio RubioOvernight Cybersecurity: Bipartisan bill aims to deter election interference | Russian hackers target Senate | House Intel panel subpoenas Bannon | DHS giving 'active defense' cyber tools to private sector Senators unveil bipartisan push to deter future election interference Puerto Rico's children need recovery funds MORE (R-Fla.) has also joined in, stumping for Mandel in March in Ohio.

The FreedomWorks endorsement came the day after a report in the Dayton Daily News alleging that Mandel gave young and inexperienced aides top positions in the treasurer's office after his 2010 election.

During that election against Democrat Kevin Boyce, Mandel repeatedly knocked Boyle for hiring people with political connections and pledged to hire the best-qualified people, "rather than political cronies and friends," the newspaper reported.

At least six campaign staffers, with an average age of 26, were brought on to the treasurer's office staff in various positions.

Mandel defended his hiring decisions to the Daily News, saying he had hired qualified financial professionals who measured up to the standards expected in previous administrations and in other states.

Polling in the race shows Brown, a first-term senator, with a lead of about 10 points over Mandel. The Hill rates this race as leaning Democratic.

- This story was posted at 4:16 p.m. and has been updated.