They also have until April 30 to change their party affiliation.
More than 3 million people are already registered to vote, according to Oregon Secretary of State data.
Of those, about 33% are registered as Democrats; 24% are registered as Republicans; 37% are unaffiliated; and the rest are registered for other groups, such as Working Families, Libertarian or Independent Party of Oregon.
But only registered Democrats and Republicans will be eligible to vote for candidates in partisan races for Congress, secretary of state, treasurer, attorney general and other legislative districts.
Voters can register , or by going to their county elections office. Ballots will be sent out about two weeks before the election.
Election day is May 21, and initial results will be posted just after 8 p.m.
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