Mitt Romney is leading Rick Santorum in Illinois, but will the underdog outperform his polls again? Meanwhile, more Americans approve of the job President Obama's doing, but a majority wants his biggest domestic accomplishment -- health care reform -- to be killed by the Supreme Court. Here's our guide to today's polls and why they matter.

 
Findings: Mitt Romney is way ahead in Illinois with 45 percent to Santorum's 30 percent.
Methodology: Robo-calls to 506 likely Republican voters on March 17 and March 18.
Why it matters: Illinois votes Tuesday. Romney hasn't been able to prove he can win very conservative voters, but Santorum hasn't been able to prove he can win anyone else. Illinois is friendly territory for Romney, The Boston Globe's Matt Viser writes, because he had a lot more money to spend on ads, and Chicago's a big city with more moderate suburban voters. On the other hand, southern Illinois should be favorable to Santorum, who's done well with Midwestern conservatives, The New York TimesNate Silver writes. Early voting indicates turnout will be low, according to The Chicago Tribune, which could help Santorum, given his more enthusiastic backers.
Caveat: Santorum has beaten his poll numbers by an average of more than two points. Other polls have showed a closer race. PPP leans left.
 
Findings: 49 percent of likely voters think the Supreme Court will strike down Obamacare, while 29 percent think it will be upheld.  
Methodology: Survey of 1,000 likely voters on March 15.
Why it matters: The Supreme Court will hear oral arguments on Obamacare March 26, and will decide whether it's constitutional before the election. The decision is expected to have a big impact on the general election. But it's not clear what the impact will be. A new ABC News/ Washington Post poll shows 42 percent want the Supreme Court to throw out the law, and another 25 percent want it to strike down the individual mandate. If the court does spike the mandate, will that relieve Obama from having to defend the unpopular measure? Or hurt him by stripping him of his signature accomplishment? 
Caveat: Many parts of Obamacare remain popular, like preventing insurance companies from not covering kids with pre-existing conditions.
 
Findings: 49 percent of Americans approve of the job Obama's doing, 44 percent disapprove.
Pollster: Gallup
Methodology: Tracking poll of about 1,500 adults over the last three days.
Why it matters: Obama's approval rating has been fluctuating a lot in recent days. Last week, some polls shows 50 percent of Americans approving of the job he's doing, while others showed him with an approval rating as low as 41 percent. Most experts say it's the trend that matters. Gallup's tracking poll shows Obama slowly inching toward positive territory. 
Caveat: Gallup's polls have tended to show Obama performing worse than most other polls this year. There's plenty of time for the president's approval ratings to drop before the general election.