The part-time, temporary employee is charged with unlawfully altering ballots; voting more than once; and official misconduct.
Authorities say another election worker claimed to see her marking a ballot that had been only partially filled out.
Clackamas County removed Swenson from her position before the election. Oregon attorney general Ellen Rosenblum says the justice department has made the investigation a high priority.
On the more serious charges, the penalty is up to five years in jail and $125,000 in fines.
The indictment is an allegation of wrongdoing. Defendants are considered innocent until proven guilty.