The national group "Senate Conservatives Fund" has endorsed Milton Wolf in campaign against Senator Roberts for the Kansas Republican primary.
Wolf is a radiologist in Leawood, Kansas, and is running against Roberts as a tea party candidate.
He's criticized Roberts on budget, tax and health care issues and sees the endorsement as helping him build momentum.
Former South Carolina Senator Jim DeMint founded the Senate Conservatives Fund.
Roberts executive campaign manager Leroy Towns says Wolf's endorsement isn't surprising, because DeMint's PAC has been endorsing challengers against incumbent GOP senators.