A former Republican judge appointed to hear arguments on whether to disqualify former President Donald J. Trump from the Illinois primary ballot said on Sunday that he believed Mr. Trump engaged in insurrection by attempting to remain in office after the 2020 election.

But the former judge, Clark Erickson, whose nonbinding opinion will be considered by the State Board of Elections on Tuesday, added that he believed the board did not have the authority to disqualify Mr. Trump on those grounds and that the question should instead be left to the courts.

The mixed decision was at least a symbolic setback for the former president, who has faced official challenges to his candidacy in 35 states and has been found ineligible for the primaries in Colorado and Maine. Mr. Trump, the leading Republican candidate for president, is still likely to appear on the primary ballots in both of those states as the U.S. Supreme Court considers an appeal of the Colorado ruling.

In Illinois, at least five of the eight members of the Board of Elections would have to vote on Tuesday to remove Mr. Trump for him to be struck from the ballot. The appointed board is made up of four Democrats and four Republicans. Their decision can be appealed to the courts before the March 19 primary.

The Illinois challenge, like those in other states, is based on a clause of the 14th Amendment of the U.S. Constitution that disqualifies government officials who “engaged in insurrection or rebellion” from holding office.

At a hearing on Friday in downtown Chicago, lawyers for residents objecting to Mr. Trump’s candidacy accused the former president of insurrection and played footage from the riot at the U.S. Capitol on Jan. 6, 2021. Lawyers for Mr. Trump denied the allegation and argued that, in any case, the constitutional clause in question did not apply to the presidency.