More than two dozen candidates filed Monday to run for former Rep. Rahm Emanuel’s (D) North Side Chicago seat, including Chicago City Council Alderman Patrick O’Connor (D), whose last-minute entrance drastically changes the contest.

O’Connor — a heavyweight among Chicago politicians and a close ally of Chicago Mayor Richard M. Daley — was supposedly on the fence about running unless he had the mayor’s backing. But he filed at the last minute with the state Board of Elections.

Other top Democrats in the contest include state Rep. Sara Feigenholtz, state Rep. John Fritchey and Cook County Commissioner Mike Quigley. Fifteen Democrats filed to run to succeed Emanuel, who stepped down earlier this month to become White House chief of staff.

The special election primary is scheduled for March 3 and the special general election will be held April 7, although the winner of the Democratic primary is all but certain to be the district’s next Representative.

Quigley’s campaign released a polling memo Monday that showed he was most well-known, with 39 percent name identification. The Anzalone Liszt Research poll also showed Quigley leading the Democratic field with 19 percent to Feigenholtz’s 11 percent and Fritchey’s 8 percent.

But when O’Connor was included, Quigley’s edge fell to 14 percent to Feigenholtz’s 10 percent. In that ballot test, O’Connor received 8 percent and Fritchey 7 percent. The Jan. 8-13 poll sampled 500 likely Democratic primary voters, and the margin of error was 4.4 points.