OLYMPIA, Wash. (AP) - Thousands of ballots being cast in Washington's presidential preferential primary are invalid because voters aren't signing a ballot oath identifying themselves as a Democrat or a Republican, Secretary of State Sam Reed said Friday.

Reed said that while some people may just forget to do it, or may be purposefully skipping voting in the presidential race and voting on special elections being held in 36 of the state's 39 counties, he thinks many are protesting the requirement that they declare themselves loyal to a party for the Feb. 19 election.

"I think this is really indicative of the attitude of Washington state voters, who are these populist voters who refuse, as a matter of principle, to be put in that box," he said.

In King County, elections spokeswoman Bobbie Egan said that of the 120,000 ballots returned so far, between 20 percent and 25 percent did not check the appropriate box on the back and sign the oath "either because voters forgot, or in protest."

Egan said each voter received an insert with each ballot that explained the rules.

"We wanted to make sure voters understood this was a unique election and there were specific rules to follow," she said. "For some voters, it's not about following instructions, it's that they're angry about the rules over the presidential primary."

Under state party rules, primary voters must pick one party's ballot and sign an oath that they consider themselves a member of that party and haven't attended the other party's caucus.

Democrats will allocate all 78 of their elected national delegates through their caucus on Saturday. The party views the primary as a nonbinding popularity contest and won't use it to allocate delegates.

Republicans will use the primary results to allot about half of their 37 elected delegates, with the other half coming from the caucus.

Most counties in Washington are conducting the primary election by mail. Voters who don't want to pick a party can still have their ballot counted for local issues such as school levies.

Earlier this week, the Kitsap County auditor's office said about a third of the 15,000 mail-in ballots it had received were invalid.

In Thurston County, officials said nearly 13 percent of the 17,000 ballots they've received so far are invalid; In Spokane, that number is just 3 percent of the 59,000 ballots so far.

Reed said he understands people's feelings, but he hopes they go ahead and do everything they need to do to make sure their ballots are valid, especially with the heightened focus on the state.

Democrats Barack Obama and Hillary Clinton were in Washington state Friday campaigning ahead of Saturday's caucus, as was Republican John McCain.

"This is such a significant race," Reed said. "Their votes should count."