U.S. Sen. Johnny Isakson clobbered two little-known Republican challengers Tuesday in his quest for a third term, while Democratic newcomer Jim Barksdale avoided a runoff against a pair of rivals in his first electoral test.

Isakson won more than three-quarters of the vote over MARTA engineer Derrick Grayson and college professor Mary Kay Bacallao, both of whom tried to outflank him on the right.

Barksdale, a millionaire investment manager recruited by party leaders, beat back opponents Cheryl Copeland, a project manager, and businessman John Coyne in his contest, which the Associated Press called shortly before midnight. He had the edge in the race after pumping more than $1 million into the race to fund ads casting himself as an outsider, but Copeland - who raised less than $10,000 for her campaign - proved a surprisingly strong challenger.

Anything short of a victory on Tuesday would have been a letdown for Barksdale's campaign, since a July 26 runoff looms if the leading candidate fails to win a majority of the vote.

Isakson is heavily favored to win in November. The 71-year-old helped build the Georgia GOP and scared off big-name Republican and Democratic challengers with a campaign war chest that hovers around $6 million.

But Democrats hope Barksdale, hand-picked by party leaders, can flip the script against Isakson. He’s trying to position himself as an outsider who can pump millions into his campaign, much like former Fortune 500 executive David Perdue did in 2014 when he won an open U.S. Senate seat.

"Everybody that makes any predictions right now at that question is guessing," Isakson said shortly after his primary victory. "I have a hunch that there’s going to be a huge turnout this November, and it’s going to be a close horse race. And it’s going to be fun to watch and fun to be in. The results are going to be the best for the people in the country."

Libertarian Allen Buckley, who forced a runoff in the 2008 Senate race, is also in the race.