Republicans largely are focused on Arizona Sen. John McCain, who could become the oldest person ever elected president, or former Massachusetts Gov. Mitt Romney, who could become the first Mormon to hold the post.
Up for grabs are 153 delegates for Democrats and 57 for Republicans.
Analysts expect interest in Obama's campaign, Chicago ward politicians' efforts and the city's large black community to help turnout, particularly among Democrats.
"I'd be surprised if the statewide voter turnout will be under 40 percent, because a lot of things are pointing that way," said Kent Redfield, a political science professor at the University of Illinois-Springfield.
Illinois traditionally hosts a mid-March primary, when early primary states typically have already combined to determine presidential nominees. This year, state lawmakers moved the primary up to give Obama an expected boost early on in the Democrats' selection process.
But daylong rain showers and evening snowfall forecast for much of Illinois could dampen some voters' enthusiasm for casting a ballot.
Even so, Ron Michaelson, former executive director of the Illinois State Board of Elections, estimated turnout could top 30 percent of the state's 7.1 million registered voters thanks to an unusually large number of truly competitive races.
That would slightly surpass the 26 percent to 29 percent turnout in presidential primary years dating to 1992, when 44 percent voted in a redistricting year that featured many races for many open seats in Congress and the Illinois General Assembly.
Cook County Clerk David Orr predicted a better than 40 percent turnout from the 1.35 million voters in Chicago's suburbs. Nearly 51,000 of them had voted in advance by late last week, with some 10,000 more expected to have done so by Election Day.
Obama, a freshman senator elected a little more than three years ago, planned to vote near his Chicago home Tuesday and hold a much-anticipated evening celebration at the downtown Hyatt Regency.
Clinton, a second-term New York senator and former first lady who grew up in the Chicago suburb of Park Ridge, televised a Monday night town hall meeting in the state and elsewhere via the Hallmark channel.
Campaign volunteers manned phone banks to get out the vote and surrogates filled in at rallies for both candidates - including Chicago Mayor Richard M. Daley, an early Obama ally, who often is neutral in primaries.
Among the high number of congressional seats drawing attention are those vacated by the retirements of Republicans Dennis Hastert of Plano, Ray LaHood of Peoria and Jerry Weller of Morris after the GOP's loss of the House majority.
In Hastert's former district, voters could in two primaries - one for a special election and the other for the general election. The GOP primary has captured much attention as Jim Oberweis, a loser in three recent statewide races, netted Hastert's endorsement and hoped to defeat state Sen. Chris Lauzen of Aurora.
As Election Day approached, Lauzen expressed some relief.
"His four months of beating the snot out of me are over," Lauzen said of Oberweis. "My guess is we're neck and neck."
Democrats have some key primaries as well, including a field of candidates trying to oust two-term Rep. Dan Lipinski of Western Springs for, they say, failing to sufficiently oppose the war in Iraq.
The Illinois GOP leadership has embraced a political newcomer - Willowbrook family physician Steve Sauerberg - in a U.S. Senate primary to name an opponent for Sen. Dick Durbin, the Senate's second-highest ranking Democrat. Sauerberg's competitors are an unemployed Chicago truck driver, Mike Psak, and freelance writer Andy Martin of Chicago.
On the state level, there are only 31 of 158 General Assembly seats up for election with more than one challenger.
