A conservative nonprofit late Monday acknowledged making robocalls in a Fairfax County Board of Supervisors race that urged voters to get out that day and vote for the Republican incumbent. but said the calls were simply a mistake and were not intended to confuse anyone.

“There was an error in the message and as soon as we recognized that, every house that received that was called back with a live operator and told the election’s tomorrow,” American Future Fund spokeswoman Mandy Fraher said late Monday.

The Democratic challenger trying to oust Republican John C. Cook from the Fairfax County Board of Supervisors’ Braddock District seat said Monday that several residents in the magisterial district have received a robocall that appears designed to sow confusion among Democratic voters.

The challenger, Janet S. Oleszek, said the early Monday robocalls told voters to go out and vote and to support Cook that day — though the election is Tuesday, and many recipients are Democrats.

“It is critically important to vote for Republican John C. Cook to protect the future of our country,” the call says. “Please go to the polls today and vote for Republican John C. Cook.”

The message — a recording of which was forwarded by Oleszek’s campaign — goes on to say that a group sounding like the American Future Fund sponsored the robocalls. The message also says the robocall was not authorized by any candidate or candidates’ committee.

Oleszek and campaign chairman Benjamin A. Tribbett say they only know of Democrats who have received the call. Tribbett said it’s possible there was a technical glitch, but it smelled more like a dirty trick. Although Virginia does not permit voters to choose a party affiliation when registering to vote, public records of their participation in the primary elections help identify likely Democrats and Republicans.

Oleszek said she was disturbed by the call and reported it to Commonwealth’s Attorney Raymond Morrogh as a possible example of voter fraud or deception intended to suppress the vote.

Cook said Monday that the robocalls disturbed him also.

“We don’t know who they are, and why they’re interested in our race,” Cook said Monday afternoon. “We’re not happy about it. . . I don’t want misinformation about that out there. That’s not good for anybody.”

This report has been updated.