Voters head to the poll Tuesday in Illinois to pick their nominees for the November ballots. The marquee matchup is the Republican gubernatorial primary, where labor has made a big investment to try to stop a candidate who appears to have the inside track.

Polls are open from 6 a.m. to 7 p.m. Central Time. Here are the four biggest things to watch:

1. Will Bruce Rauner survive labor's onslaught in the governor's race?

Organized labor came together to spend big money trying to prevent former private equity executive Bruce Rauner from winning the GOP nomination. But their effort looks like it is on the verge of falling short. Rauner spent big in his own right, and polls show he is the frontrunner headed into the vote. Labor spent more than $3.6 million opposing Rauner, who said public unions are "organized against the public good." Labor has also tried to boost state Sen. Kirk Dillard (R), Rauner's closest competitor and an opponent of a key pension reform law. State Sen. Bill Brady and state Treasurer Dan Rutherford are also running. If Rauner wins, it will set up a interesting general election contrast between a longtime government official and a first-time candidate.

2. Pat Quinn's noncompetitive primary

We've written about this before, but it bears repeating: Gov. Pat Quinn (D) faces only token competition for the Democratic nomination Tuesday and is expected to cruise to victory. Considering how unpopular he was headed into the 2014 cycle, and the fiscal mess the state was facing, it's nothing short of remarkable that he does not find himself with a real race on his hands. How'd he do it? A combination of luck and skill. He got lucky when Attorney General Lisa Madigan (D) decided not to challenge him. And by going on offense early against former Obama White House chief of staff Bill Daley, who did run, Quinn deserved some credit for Daley bowing out early. None of this is to say Quinn is safe in November. He remains a top target for Republicans. Beginning Wednesday, things are going to get a lot tougher for him. 

3. The three state House Republicans who voted for gay marriage

Illinois legalized gay marriage last year after a bill passed that was supported by just three Republicans in the state House. (The law will go into effect this summer.) Those three are under the spotlight Tuesday, with two -- state Reps. Ron Sandack and Ed Sullivan -- running in primaries in which gay marriage has been an issue and one running for the GOP nomination for state treasurer, state Rep. Tom Cross. The debate over gay marriage laws may be settled in Illinois, but Tuesday will test what, if any political costs the Republicans who voted for the measure will face.

4. The 13th congressional district

Don't expect any incumbents to fall in U.S. House primaries Tuesday. But if there's one district to watch it's the 13th, where Rep. Rodney Davis (R) faces a primary challenge from attorney and former Miss America Erika Harold (R). Davis is expected to advance and Harold never really emerged as a threat. But it's still worth keeping an eye on this race, which Democrats are expected to contest heavily in the general election. Democrats' recruited former judge Ann Callis, who is expected to clinch the nomination Tuesday but faces some minor competition. The House landscape in Illinois will become more interesting in the general election, with Reps. Brad Schneider (D), Bill Enyart (D) and Cheri Bustos (D) also expected to face competitive contests.