U.S. Sen. Johnny Isakson's supporters have formed a super PAC to help his campaign for a third term - and send a message to any would-be challengers.

The Revitalize America PAC has so far only raised $12,500 to help the Georgia Republican fend off any challengers, but that sum is sure to grow as the November 2016 vote nears.

Democrats are struggling to find a star candidate to run against the two-term incumbent, and many of the top candidates in last year's race have already bowed out. That means Isakson's bigger concern may be a challenge from his party's right flank.

The 70-year-old is sitting on nearly $4 million to ward of potential rivals, and he raised $1.6 million in the first three months of the year. By comparison, at this point in his last re-election cycle, Isakson raised just $379,000 and had $2.47 million on hand.

The super PAC is not bound by the cap on individual contributions that ties traditional political campaigns, and it also cannot coordinate with Isakson's camp. It was created in May 2014, but was only recently shifted to support Isakson.

The fund's supporters include Lt. Gov. Casey Cagle and former Sen. Saxby Chambliss, said Anne Lewis, the fund's legal counsel.

"The focus of the PAC is to re-elect Johnny Isakson because he's served the state very well," she said.