If early voting is any clue, Valley turnout in today's election will be big.

The tight presidential race and litany of down-ballot contests, including Gov. Jerry Brown's tax measure that brought him to Fresno on the stump Monday, has prompted record numbers of early voters -- people who cast their ballot by mail or at an election office that opened early.

More than 100,000 people had voted in the central San Joaquin Valley as of the weekend. In Fresno County, nearly 80,000 had mailed in their vote.

Elections officials warn, however, that the high volume of votes, particularly from mail ballots, could make counting more cumbersome and mean verdicts in close contests won't be known tonight.

More than half of Fresno County's roughly 208,000 mail ballots were yet to come in as of Monday morning, according to elections officials, meaning the bulk were still en route and likely would be counted after the busy Election Day.

It was a similar situation across the state. With half of Californians voting by mail and many waiting until the last minute to drop off ballots, delays in the count are widely expected.

Voter interest has been heightened not only by the high-profile ballot but by the conveniences of online voter registration and mail voting, which has surged since California law changed to allow anyone to cast a mail ballot.

Voter registration hit an all-time high last week of 18.2 million voters statewide, the Secretary of State's Office reported.

While elections officials are reluctant to project turnout, many concede that it won't be as high as it was in 2008. State turnout four years ago -- 79.4% -- was the highest in three decades.

Fresno County Registrar of Voters Brandi Orth said her office is trying to make it as easy as possible to cast a vote, offering nine drive-through spots to drop off ballots today and employing close to 2,000 temporary workers to help out with elections today.

A warm, sunny day in the Valley also is expected to boost turnout.

Glenn Lostracco of Fresno wasn't about to take any chances at the polls. He dropped off his ballot Monday morning at the county elections office, where a steady stream of people submitted votes.

"I don't decide how to vote at the last minute," Lostracco said. "I usually send it in earlier, but I've been rather busy so I haven't had the time."

With the presidential race in California all but locked up for President Barack Obama -- due to the state's large number of Democrats and winner-take-all electoral system -- Lostracco said he's most interested in the statewide propositions.

Brown's initiative to raises sales and income taxes to pay for schools and public safety is one that concerns him, and abolishing the death penalty is another.

Rachel Derby, who also dropped off her ballot Monday, said she was looking forward to weighing in on a Fresno County tax measure to fund libraries, Measure B.

"I have to work Tuesday so this is the best time for me to vote," Derby said.

Other parts of the Valley saw similarly high numbers of people voting early.

"It's been going briskly," Tulare County elections chief Rita Woodard said.

Like in Fresno County, voting in some parts of Tulare County is limited to mail ballots, adding to the number of people voting before Election Day. State law allows elections officials to forego polls in smaller precincts.

In Kings County, elections officials experienced a wrinkle in mail voting that they're hoping won't affect turnout. About 500 voters did not receive their requested mail ballot due to a computer glitch, according to Registrar of Voters Ken Baird.

The elections office noticed the problem on Thursday and has been notifying those affected and trying to get ballots in their hands.

"We think we've made contact with as many people as we can, and we think the problem is as resolved as it's going to be," Baird said.