The anti-tax Club For Growth is nowhere to be found. Karl Rove's American Crossroads hasn't made a peep. Establishment agitator Senate Conservatives Fund/Action? Radio silence.

The Republican primary for U.S. Senate in Georgia has caught the attention of super PACs -- but not the usual heavy hitters that typically muscle into primaries with tea party-vs.-establishment battle lines. Instead, obscurer cousins like Ending Spending Action Fund and Citizens for a Working America have populated the air space. In a crowded race most watchers predict will head to a top-two runoff on May 20, that dynamic is mostly good news for Republicans hoping to nominate an electable contender. It's especially good news for businessman David Perdue, who has not taken any blows on the air from a third-party group yet.

In this Jan. 18, 2014 file photo, Republican candidate for U.S. Senate David Perdue speaks during a debate at the Cook County High School Performing Arts Center in Adel, Ga. (AP Photo/Phil Sears, File)

A super PAC saying it backs Perdue has hit the air, but so far, no super PAC opposing him has done so. Citizens for a Working America PAC, not a factor in any other 2014 race thus far, is slamming another GOP candidate, Rep. Jack Kingston. It says its goal is to help Perdue. Perdue is also helping himself by spending big on his own ads.

While Kingston is taking heat, he has the well-heeled U.S. Chamber of Commerce in his corner. The chamber has not hit the airwaves in Georgia yet but has spent big for candidates in other races, suggesting it will be there for the congressman. Karen Handel, the former secretary of state, has the support of the American Future Fund. Like the chamber, AFF has not gone up on the air, but has shown a willingness to spend money in prior races.

Kingston, Handel and Perdue are viewed by many Republicans as more electable alternatives to Reps. Paul Broun and Phil Gingrey. Neither Broun nor Gingrey have super PACs hitting the airwaves on their behalf, even as they have each received high marks from conservative groups, such as the American Conservative Union and the Club for Growth. Broun sports a 99 percent lifetime rating on the Club's scorecard; Gingrey boasts a respectable 86 percent. Both were in the Club's top 20 House voting records in 2013. Neither has a Club endorsement.

"We are watching the race," is all Club spokesman Barney Keller would say when asked about the group's strategy.

Both candidates could use a boost. Broun has scant campaign cash, and Gingrey is taking incoming attack ads from Ending Spending Action Fund, a super PAC that was founded by billionaire TD Ameritrade founder Joe Ricketts and has also attacked Democratic candidate Michelle Nunn. Super PACs can raise and spend unlimited sums and must disclose their donors.

Broun's knack for controversy -- he once accused President Obama of upholding the "Soviet constitution" and called theories of evolution and the Big Bang "lies straight from the pit of hell" -- has stoked Republican fears about falling to potentially formidable Nunn. Gingrey's potential to go off-script -- he suggested that failed 2012 Senate candidate Todd Akin's controversial comment that "legitimate rape" rarely causes pregnancy was "partly right" before later walking it back -- has raised similar concerns.

The Senate Conservatives Fund, which is backing conservative insurgents in other Senate races, has also stayed out of the Georgia mix. The tea-party aligned Madison Project is backing Broun, but has not hit the airwaves on his behalf. (TV is a highly effective medium for reaching voters in a populous state like Georgia, which is why it's so important.)

American Crossroads, a super PAC whose president has voiced concerns about nominating unelectable candidates and has run positive spots for Senate candidates engaged in primaries in North Carolina and Alaska, has not jumped into the Georgia mix. Crossroads won't say what it intends to do there. But its absence has fueled the argument that for now at least, the most electable GOP candidates are not in dire need of help.

If the election were held today, most observers feel that Perdue and Kingston would be best-positioned to make an all-but-certain runoff. If no candidate receives a majority of the May 20 vote -- and no candidate is polling anywhere near 50 percent right now -- the top two would face off in July.

If the runoff ends up being Kingston vs. Perdue, Republicans with an eye on the majority will be breathing a little easier.