SEATTLE — If there was any doubt, Election 2012 has confirmed it: Washington is the slowest vote-counting state in the country.

As of Thursday night, 30 states and the District of Columbia had counted virtually 100 percent of their ballots, 12 states had tabulated 99 percent and everybody else — including Washington’s vote-by-mail sibling, Oregon — were hovering around 95 percent or higher.

Washington state had tallied 76 percent, with at least three major contests still hanging in the balance: the governor’s matchup, the race for secretary of state and a ballot measure on whether to allow charter schools.

As in the past, King County — the state’s largest — was no exception, with just more than 70 percent of its expected 1 million ballots counted. It counted about 100,000 ballots Thursday, most received Tuesday, and it still had roughly 275,000 more to count.

Elections officials blamed voters who waited until the last minute to cast their ballots and laws allowing them to do so. In this state, voters need only to postmark their ballots by Election Day, meaning that ballots often trickle in long afterward. On Thursday, King County reported receiving some 5,000 new ballots.

From receipt to tabulation, bundles of ballots take about a day and a half to process, mostly because of security requirements such as manually checking signatures against voter files, officials said. If a ballot has a problem — a missing signature or the wrong ink color — it takes longer.

Counting faster would be costly, said Katie Blinn, the state’s co-election director.

In Oregon, which had counted 95 percent of its expected votes by Thursday, ballots must be received by county election offices by Election Day. The state, which has a smaller population than Washington and has been doing all-mail voting since 2000, uses electronic signature verification and asks election workers to work late into the night, said Andrea Cantu-Schomus, a state spokeswoman.

In King County, workers go home at 8:30 p.m. or earlier.

“Here’s what I know about working people until midnight and having them back early in the morning: That’s when you have problems,” said Sherril Huff, director of King County Elections. “That’s when you make mistakes.”

Huff was clearly frustrated by days of complaints about the slowness.

“We have the largest voter population in this state,” she said, noting that King County has twice as many registered voters as the next largest county, Pierce. “What is it about that people don’t understand?”

King County hired 500 temporary workers, including 300 ballot counters, and they work in pairs. That’s not much more than Pierce County hired, said Huff, explaining that the King County Elections building did not have space for any more workers.

The county also has been slowed by ballot scanners that, apparently stressed by a high number of ballots, broke down Wednesday and last Saturday, Huff said.

Officials were able to reboot the machines quickly, but starting them takes an hour because of security protocols, she said.

“If people want faster ballot counts, they need to move to Oregon,” she said.