Johnson said Tuesday that he made the decision in August, but declined to elaborate about why except to say he has been concentrating on development and fundraising.
He said he is committed to the race in which Radtke faces former governor and senator George Allen, businessmen Tim Donner and David McCormick and Bishop E.W. Jackson.
“I fully intend to stay here,’’ he said. “I’m still here helping the campaign.”
Meanwhile, Radtke is searching for a new campaign manager as she struggles to gain traction in the Republican primary dominated by Allen.