SACRAMENTO — Election experts said Monday they expect about 56 percent of voters to cast ballots in today's presidential primary — the highest turnout in nearly three decades — and warned that the final tally may not come until late Wednesday morning.

Several factors are driving the projection, made by the California Association of Clerk and Election Officials.

For one, the presidential contest in both parties is close enough to spark huge interest, reflected in record numbers of registered voters. Also, a higher percentage than ever of those voters have requested absentee ballots. Absentee voters are more likely to follow through and vote than those who vote at polling places.

Mark Baldassare, director of the Public Policy Institute of California poll, is among those who expects today's turnout to be from 55 percent to 60 percent. That would be in the neighborhood of the percentage of voters who cast ballots in the 2003 recall. It would also be the highest turnout for a presidential primary since 1980, when 63 percent of registered voters cast ballots.

Pollsters, analysts and government officials said they expected the highest turnouts in Bay Area counties, as usual.

Baldassare said interest in the primary was fueled by the state's early timing, "uncertainties" facing a nation at war, and the "enthusiasm" of a race for the Democratic nomination between a woman and an African American.

It's also the first time since 1952 that no incumbent president or vice president is running.

"People are thinking their vote counts," Baldassare said, "and that they are participating in history."

The state's top election official — in an unusual move — decided not to issue a turnout prediction, said Nicole Winger, a spokeswoman for Secretary of State Debra Bowen.

"There's no way to predict with the unique factors," Winger said.

The state moved its presidential primary up from March or the traditional June date in hopes of having more impact on the outcome of the race. The election coincides with primaries in 23 other states.

Winger noted that voter registration has hit a record for a presidential primary, with 15.7 million voters, or about 68.5 percent of those eligible to vote. Included in that total is a record percentage of voters, nearly

20 percent, who have declined to state their party preference, and a record number of mail-in ballots.

The former could weigh heavily in the Democratic primary, which allows decline-to-state voters to participate. The Republican party only allows GOP members to vote in its primary. The latter could create delays in deciding races, especially if a large number of those ballots are returned or received on election day, which means they must be verified before they can be counted.

Guy Ashley, of the registrar of voters office in Alameda County, said a large turnout is expected locally, of 70 percent to 75 percent of the county's 704,000 registered voters.

Ashley said the trend toward mail-in ballots has affected when results can become available. In Alameda County, nearly 50 percent of the total turnout for elections vote via mail-in ballots.

"Mail-in ballots used to just be a trickle of the votes that came in," Ashley said. "Now, however, it's a big thing."

Ashley said the registrar's office won't be able to start counting mail-in ballots received on election day until Wednesday due to the large amount of ballots cast at polling locations the office will count today. Ashley said those mail-in ballots received today could represent a "sizable chunk" of mail-in ballots received for the early presidential primary — possibly as much as 15 percent to 20 percent.

Testing by Bowen's office last year also concluded that many electronic voting machines were vulnerable to hacking, which force more than 20 counties to switch back to paper ballots. That development has caused a shortage of the high-speed optical scanning equipment that's required to process the ballots swiftly. 

Contra Costa County election chief Steve Weir said that because his county traditionally uses paper ballots and optical scanning equipment, he is not expecting major problems.

Staff writer Chris Metinko contributed to this story.

Contact Steve Geissinger at sgeissinger@bayareanewsgroup.com or 916-447-9302.