"I think vote-by-mail is here to stay," Secretary of State Bill Bradbury said a day after an election in which 50 percent of voters participated.

The turnout was the highest for an Oregon primary election since 1988, when 55.1 percent of voters cast ballots. In the last primary election in 1998, 34.9 of registered voters participated.

Vote by mail was mainly responsible for the increase, Bradbury said.

"I don't see how you could conclude otherwise," he said. "The election wasn't exciting and you have to say, `There must be other factors here."'

The figures were unofficial and don't include a small number of unreadable ballots or others that were misdelivered. Although critics have said mail balloting could invite attempts at fraud, State Elections Director Colleen Sealock said no problems were reported.

Oregonians have voted by mail in local elections and special statewide elections since the early 1980s, but Tuesday's election was the first full-blown primary conducted exclusively by mail since Oregon voters abolished polling places two years ago.