With nearly all precincts reporting Tuesday, Alvin Greene, 32, commanded 59 percent of the vote against 41 percent for former four-term state lawmaker Vic Rawl, 64, who had raised about $186,000 and had to abruptly scrap a late-week fundraiser for the fall.
State Democratic Party Chairwoman Carol Fowler said voters unfamiliar with either candidate may have voted alphabetically for Greene over Rawl.
"As far as I know, he never showed up at anything. Vic Rawl has been campaigning everywhere from the time he filed," she said.
Rawl said he was disappointed.
"I would've liked very much to be a candidate against Jim DeMint," Rawl said, describing his sole primary rival as something of a mystery. "I never saw him. I've still never met him."
As for Greene, he couldn't explain it either but thanked voters in a state numb with high unemployment and said: "Let's continue to make history and get South Carolina back to work."
Greene said he spent a total of 13 years in the Air Force and Army before leaving the Army in August.
DeMint, a conservative Republican pursuing a second term, has marshaled a $3.5 million war chest already to face the bare-pockets Democratic underdog.
Political scientist Scott Huffmon at Winthrop University said the looming DeMint-Greene contest already shapes up in lopsided favor of DeMint and shows South Carolina Democrats lack depth to field strong candidates in every race.
"DeMint's coasting pretty much to re-election," he predicted.
Late Tuesday, stunned Democratic leaders in South Carolina struggled to comprehend how the little-seen candidate upstaged Rawl, a moderate Southern Democrat they viewed as their far stronger bet against DeMint. Rawl's lengthy resume lists four past state House terms and former posts as prosecutor, circuit court judge and more.
DeMint trounced a Charleston lawyer, Susan Gaddy, in the Republican contest to advance.
More CBSNews.com Coverage of Tuesday's Primaries:
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Women Win Big on Primary Day
Interactive Map: CBS News Campaign 2010 Race Ratings