The man, who posts as Mike Mazzone on Twitter, had donated $1,000 to Ives' campaign. His Twitter profile features a picture of him giving a Nazi salute.
In a statement released Sunday, Ives said the staffer didn't realize who the man was when they re-tweeted his post about the contribution.
The Ives campaign is returning the donation. Ives has previously been criticized for her controversial ads attacking Governor Bruce Rauner's stances on abortion, immigration and transgender rights.