Former governor George Allen has hired Steve Waters, a social and fiscal conservative activist who managed Bob Marshall’s 2008 Senate race, as he looks to burnish his conservative credentials in the 2012 Senate race.

Waters had been critical of Allen in recent months, but said in an interview that he came to believe the former senator is the the only one who can take on former Democratic governor Tim Kaine in a general election.

Some conservatives and tea party activists accuse Allen of abandoning right-of-center values, backing big-government programs and too much spending, “He’s expressed a willingness to work on these issues,’’ Waters said.

Waters begins his full-time paid position as adviser on family policy issues May 1.

“Steve’s experience in organizing and educating grassroots activists will be valuable to our grassroots insurgency as we take our positive, motivating message to men and women throughout Virginia,’’ Allen said in a statement.

Waters has long been a supporter of Marshall, a delegate from Prince William, who is still contemplating a Senate run next year, and said he told him of his decision.

Wednesday night, Marshall said he was “a little surprised” by Waters’ decision, and that Waters never told him he was taking the job.

“You can tell who the candidates are not by where the consultants go, but where the volunteers go,” Marshall said.

Waters said he was motivated to get involved after he learned that Kaine, then governor, proposed a change in adoption regulations in November 2009, less than two months before he left office to force faith-based groups to no longer discriminate in adoptions. He praised Allen for leading on parental notification of abortions for minors and banning of partial birth abortions.

Allen faces a slew of Republicans in the June primary: Jamie Radtke, former chairwoman of the Virginia Tea Party Patriots Federation; Hampton Roads lawyer David McCormick, Bishop Earl Jackson and wealthy political neophyte Timothy E. Donner.

Corey A. Stewart, chairman of the Prince William County board of supervisors; Bert Mizusawa, a businessman and lawyer who ran against U.S. Rep. Scott Rigell and Marshall are considering a run.

Marshall almost defeated former governor James S. Gilmore III to capture the Republican nomination for U.S. Senate at a 2008 convention to run against former Democratic governor Mark R. Warner. The margin was less than one percentage point.