Chicago-area voters head to the primary polls Tuesday to select former Rep. Jesse L. Jackson Jr.’s successor.

Among the Democrats in the race, Cook County Chief Administrative Officer Robin Kelly is the candidate most political observers expect to win, but special elections are unpredictable. There are narrower paths to victory for former Rep. Debbie Halvorson and Alderman Anthony Beale.

Adding to the uncertainty in the 2nd District special election is a crowded lower tier of candidates. The winner of the Democratic primary is all but certain to win the general election in this heavily Democratic district.

New York Mayor Michael Bloomberg injected himself — and his money — into the race, with a multimillion-dollar negative ad campaign blitz against Halvorson. Halvorson countered Bloomberg’s ads by painting him as an outside interloper.

Bloomberg’s point of contention is Halvorson’s position on gun control, and liberal groups are quick to note that Bloomberg’s move has overshadowed their efforts. The Daily Kos raised $140,000 on Kelly’s behalf, the Service Employees International Union flooded its members in the district with anti-Halvorson direct mail and a group called Progressive Kick is claiming partial credit for pushing state Sen. Toi Hutchinson out of the race.

Jackson resigned in November and pleaded guilty Feb. 20 to a felony count of using campaign money to fund personal expenses.