Deanna Swenson, 55, of Oregon City, has been charged with altering a cast ballot, unlawfully voting more than once and official misconduct, Attorney General Ellen Rosenblum announced Thursday.
Swenson had yet to be taken into custody. She did not return a message left on her answering machine and it is unknown if she has hired a lawyer.
Investigators found no evidence that others were involved.
Swenson, a temporary employee who had worked for the county in prior elections, is accused of filling in ovals on ballots where preferences had been left empty by voters.
The alleged misconduct occurred Oct. 31, less than a week before the general election, when another election worker claimed to see the registered Republican marking a partially complete ballot. It is common for voters to make choices in high-profile races and then skip the local contests in which the names of the candidates are unfamiliar.
Oregon was the first state to conduct elections exclusively by mail, and the November election marked the fourth presidential contest in which the system was used. Never before had an election worker been accused of marking choices.
"The integrity of the Oregon elections is of paramount importance," Rosenblum said. "The Department of Justice has made this investigation a high priority since the allegation came to light."
The indictment did not specify the extent of the alleged tampering. Andrea Cantu-Schomus, spokeswoman for Secretary of State Kate Brown, said election officials identified six suspect ballots. Rosenblum's office, however, declined to say how many ballots were altered, citing the pending trial.
The alleged tampering apparently did not affect any races. All the suspect ballots were reviewed to identify voter intent - or, in this case, lack of intent - and then counted.
Brown plans to certify the election results Dec. 7.
"Security procedures are in place to detect this kind of conduct," said Brown, who responded to the allegation by assigning two people to monitor ballot processing in Clackamas County. "The system worked and no person's ballot was impacted. Oregonians can be confident in their election results."
Altering cast ballots and voting more than once are felonies punishable by up to five years in prison. Official misconduct is a misdemeanor, punishable by up to a year in a jail.