Sonoma County has issued nearly 185,000 vote-by-mail ballots for Tuesday's election, a record that represents 71 percent of registered voters.

But it also means any close race, such as the 1st District supervisorial contest between Santa Rosa City Council members Susan Gorin and John Sawyer, may remain unresolved for up to four weeks after the election.

The burgeoning number of absentee voters, along with high voter participation and a lengthy ballot in the presidential election, virtually guarantees officials will need the full 28 days allowed by law to tally the vote, county elections chief Janice Atkinson said Thursday.

"It will take everything we can do to make that" deadline, she said.

One of the major delays will be the painstaking process of verifying an estimated 35,000 vote-by-mail ballots expected to arrive in Tuesday's mail or turned in at polling places Tuesday.

Only vote-by-mail ballots returned by 5 p.m. Monday can be counted on Election Night, along with the traditional ballots cast at the polls on Election Day, Atkinson said.

She is expecting about 186,000 votes to be counted on Election Night, based on an estimated 85 percent overall turnout by county voters.

If her forecast is correct, nearly 16 percent of the total vote -- the 35,000 vote-by-mail ballots delivered Tuesday -- will not be counted for at least three weeks after Election Day, potentially clouding the outcome of races with razor-thin margins.

In the June primary, about 23,250 vote-by-mail ballots came in on Election Day, and it took 22 days to report the county's final results, including Gorin's 186-vote advantage over Sawyer.

With 184,671 absentee ballots issued and 260,448 registered voters, Sonoma County had one of the highest rates -- 71 percent -- of absentee voting in California.

Neighboring Napa County had 86 percent, Mendocino County 80 percent and Lake County 61 percent, compared with a statewide absentee ballot rate of 50 percent, according to the latest report on Thursday.

"It's a different ballgame," said Valerie Brown, the outgoing Sonoma Valley supervisor.

In 2008, when she won her third term on the board, Brown said, she was on "tenterhooks" for three weeks awaiting the final results -- a delay attributed to high voter turnout and mail-in ballots arriving on Election Day.

Herb Williams, a veteran Santa Rosa political consultant, said he finds no fault with the registrar's office in taking weeks to determine election results.

"I like the integrity of the process," he said, referring to the verification of signatures on vote-by-mail ballots.

Atkinson said the verification process is partly automated but also involves a visual comparison of signatures on the mail-in ballots with those on file in voter registration records.

The process could be expedited, she said, but Sonoma County makes a major effort to validate every possible mail-in ballot.

Back when most people voted at the polls, election results were tallied much quicker, said Atkinson, who is retiring in December after 36 years in the elections office.

Tom Higgins, a Marin County political consultant, said the liberalizing of vote-by-mail standards over the past 20 years has led to slower vote counts in California. Sonoma and Los Angeles counties were the last two to complete their counts in June, he said.

But promoting mail-in voting is "absolutely the right thing to do," Higgins said, and it's not just procrastination that causes the flood of absentee ballots on Election Day.

Some absentee voters intentionally wait until the last day to take account of "late developments" in a campaign, he said.

You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.