On the eve of today's vote, Riordan criticized Gov. Gray Davis for "hijacking the Republican primary" by funding attack ads he claims have helped rival Bill Simon enjoy a late surge in the polls.
In another high-profile race, Rep. Gary Condit hoped to fend off the well-funded challenge of a former aide Tuesday in the Democratic primary for his congressional district.
Simon, son of the late treasury secretary under Presidents Nixon and Ford, went from obscurity to first place in recent GOP polls to surpass Riordan in the race to take on Davis.
Davis, a Democrat with no serious primary opposition, launched an unprecedented $10 million effort to bruise Riordan, the moderate Republican candidate he sees as his biggest potential threat to unseat him in November.
"We can't let him get away with that," Riordan told supporters at a breakfast stop during a tour of the state Monday. Riordan had been the favorite for months.
Many analysts say Riordan has a better chance of beating Davis in the general election because his support for gun control and abortion rights are agreeable to voters in the heavily Democratic state.
A Field Institute poll late last month showed Simon overcoming a 33-point deficit to emerge six percentage points ahead of Riordan, while more recent polls conducted by the campaigns show an even larger gap.
Secretary of State Bill Jones, who trails far behind Simon and Riordan, has said he expects turnout for Tuesday's vote to be low — about 36 percent of the state's more than 15 million registered voters — which could help Simon.
The White House is closely monitoring the campaign, which has widened the fracture in the state Republican Party. Riordan attracts GOP moderates who believe the party needs to be more inclusive, and Simon has won the backing of loyal conservatives.
The cordial campaign between Simon and Riordan, friends who attend the same Los Angeles parish, dissolved into sharp criticism and negative television ads after polls showed Simon gaining on and eventually surpassing Riordan.
"I still consider Dick a friend," Simon said. "I know in the closing days of a campaign the rhetoric often gets heated up."
Meanwhile, Condit tried to bolster support Monday for his bid to hold the 18th Congressional District against a challenge from his main opponent, Democratic state Assemblyman Dennis Cardoza.
The winner of the primary will face the victor in a four-candidate GOP field for the reconfigured district.
Condit, his political career in tatters since the disappearance in May of Washington intern Chandra Levy, has within the last week sent voters in his district a letter asking for a "chance to regain your trust and redeem myself."
Washington police sources have said Condit admitted he had an affair with Levy, although in media interviews he has refused to reveal the nature of their relationship. He is not a suspect in her disappearance.