The process of winnowing Chicago’s crowded mayoral field began in earnest Monday, as candidates began fighting to bump other contenders off the February ballot.

As of late afternoon, at least half of the 21 people vying to be the city’s next mayor had their petitions challenged. To see the most updated list of challenges to mayoral and aldermanic candidates, click here.

Illinois’ arcane election laws dictate that mayoral candidates must gather at least 12,500 valid signatures from registered Chicago voters in order to appear on the Feb. 26 ballot. Aldermen need 473.

Starting Dec. 10, candidates for mayor, citywide offices, and alderman will begin hearings to knock their opponents below the signature threshold needed to appear on the ballot. To do that, campaigns will accuse their opponents of everything from gathering forged signatures, to filing paperwork the wrong way, or other menial misdeeds that run afoul of Illinois’ finicky election laws.

Democratic Cook County Board President Toni Preckwinkle acknowledged Monday morning that she is behind a challenge to another reputed front-runner in the mayoral race, Illinois Comptroller Susana Mendoza.

The co-chairs of Mendoza’s campaign, Marty Castro and Kathy Byrne, held a press conference Monday morning accusing Preckwinkle and two other candidates, Bill Daley and Gery Chico, of working together to knock Mendoza off the ballot.

“I was on the (Illinois) Racing Board for years and I know a little bit about horse races and I know that the absolutely best way to keep the favorite from winning the race is to lock the horse in the barn, and that’s what they’re trying to do,” said Byrne, the daughter of former Mayor Jane Byrne.

Byrne painted Mendoza as the front-runner and a threat to the status quo. Preckwinkle is chairman of the Cook County Democratic Party, and Daley comes from the Chicago political dynasty that has created two mayors.

“Let the people decide if she has what it takes,” Byrne told reporters. “The lawyers and the party bosses shouldn’t decide.”

While the Preckwinkle campaign early in the day admitted to challenging Mendoza, Chico and Daley said they were not involved. When asked, Daley’s spokesman, Peter Cunningham, responded with a simple, “Not true.” And Chico spokeswoman Kelley Quinn said they’ve not communicated with any other campaign.

“We did not challenge anyone, nor did we get challenged,” Quinn said.

Despite her criticism, Mendoza herself has challenged other candidates’ petition signatures in her own political career.

Ja’Mal Green, the youngest candidate at 23 years old, accused Willie Wilson of being behind a challenge to his petitions. Green’s campaign said Wilson, who took third in the 2015 election, is targeting other African-American candidates. Wilson’s campaign had no comment.

Ambriehl Crutchfield contributed reporting for this story.