The special election is to fill the seat of Jackson Jr, who resigned last November citing health problems, and pleaded guilty in federal court last week to using campaign funds for personal enrichment.
Bloomberg has poured more than $2 million from his political war chest to support former state representative Robin Kelly, who supports tighter gun restrictions.
The Illinois State Rifle Association, a lobbying group aligned with gun rights group the National Rifle Association, has endorsed Debbie Halvorson who opposes an assault weapons ban.
Gun control vaulted to the top of the U.S. political agenda after the December 14 killing of 20 children and six adults at an elementary school in Newtown, Connecticut.
The election outcome will be an indication of whether Bloomberg and other advocates of gun control can effectively challenge the money and political influence of the NRA, which has long been a powerful force in U.S. elections.
Jackson Jr. was a reliable vote in Congress for gun control. But early polls in the race to succeed him showed that the Democratic primary could be won by Halvorson, who has an "A" rating from the NRA.
Bloomberg elbowed his way into the race, blanketing Chicago television with ads attacking Halvorson and endorsing Kelly.
Turnout was expected to be light in the special election and could be hurt by a snowstorm that hit the Chicago area about midday on Tuesday, making travel treacherous.
Kelly made one last push to gather votes Tuesday at an International House of Pancakes restaurant in the suburb of Matteson.
"We are driving the people that need rides (to the polls). ... Whatever they need, my troops are out doing that," she said.
With 14 candidates in the race, the outcome is difficult to predict. One public poll published last Friday showed Halvorson holding a slim lead, but some private polls have showed Kelly pulling ahead.
The large field of candidates drew some complaints from voters accustomed to Jackson Jr., who represented the district for nearly two decades.
"We've got to start getting together. We've got too many people running," said Donald Massingale, 73, a longtime Jackson Jr. supporter.
Some voters also said they were glad the special election was being held to fill the seat. Jackson Jr. disappeared from public view last June to be treated for mental health issues, and the district has effectively been without a representative in Washington since then.
"I don't think the people really understand the importance of this race to have the representation that our district needs," Delores Evans, 58, a poll worker said as she voted before starting work.
The race could also be affected by Chicago's racially charged politics. The district is majority African-American although parts of it stretch south to the predominantly white outer suburbs of Chicago.
Halvorson, who is the only white candidate for the seat, has appealed to suburban voters who might be sympathetic to gun ownership. Kelly, who is black, has highlighted a plague of gun violence in Chicago's inner city.
The winner of the Democratic primary is likely to be elected to the seat because the district is overwhelmingly Democratic.
(Reporting by Renita Young, writing by Greg McCune, editing by Jackie Frank, desking by G Crosse and Richard Chang)