Leading the pack were former congressman-turned-lobbyist Brian Bilbray, the choice of the GOP establishment; wealthy businessman and Republican Eric Roach, a political newcomer who has spent $1.8 million of his own money; and Democrat Francine Busby.
They are seeking to fill the House seat in the solid GOP district on the Southern California coast.
Cunningham represented California's 50th Congressional District from 1993 until he resigned in disgrace late last year. In March, he was sentenced to more than eight years in federal prison on charges of evading taxes and accepting $2.4 million in bribes from defense contractors.
Tuesday's vote, an everyone-on-the-ballot format involving 14 Republicans, two Democrats, one Libertarian and one independent was the first of four congressional ballots expected in the district this year.
If no candidate gets more than 50 percent of the vote, the top finisher from each party competes in a June 6 contest, the same day as the primary for the midterm congressional elections.
An outright winner Tuesday or, more likely, the winner of the June contest takes a seat in Congress for the remaining eight months of Cunningham's term — and likely immediately begins campaigning to hold on to the seat come November.
Republicans outnumber Democrats 3-to-2 in the wealthy district. With the GOP field crowded, Democrats hoped that Busby, who ran and lost to Cunningham in 2004, could score an upset, a prospect that her own campaign acknowledged was unlikely.
Voters faced an onslaught of negative campaign mailers and TV ads in the final hours before heading to the polls.
Bilbray, a surfer and one-time lifeguard, was attacked in a mailer from the California Republican Assembly, a grass-roots conservative group, that called Bilbray "a Democrat posing as a Republican."
The Bilbray campaign attacked Roach as a mystery man trying to buy an election. "His political campaign ads and mailers are everywhere. But until a few months ago, no one had ever heard of him," Bilbray's mailer said.
Former state Assemblyman Howard Kaloogian's campaign attempted to recover from an embarrassing misstep. Internet bloggers discovered that photos on his campaign Web site purportedly showing a tranquil Iraqi street scene had been taken in Istanbul, Turkey. The photos were removed.
With the GOP field busy attacking itself, national Republicans poured some $300,000 during the campaign's final week into TV ads criticizing Busby for accepting donations from lobbyists. A mailer from national Republicans claimed that the school board member and self-described soccer mom had voted to lay off teachers in 2003.
Busby retaliated by noting that Cunningham had given $2,000 of his campaign funds to national Republicans before he headed off to prison.