Two states that are being talked about the most at the moment are Ohio, Florida.
It's Primary Day a.k.a. Super Tuesday #3 in America - here's how you can livestream the results in Florida, Illinois, Missouri, North Carolina and Ohio.
Kasich prevented a total Trump takeover by denying him victory in Ohio.
Sanders tops Trump in the state by nine points, 50 percent to 41 percent, and Cruz by one, 45 percent to 44 percent.
And Kasich, with 63 delegates, is betting that a win in OH will keep his campaign afloat. Even in the face of these discouraging calculations, the Republican governor is not deterred.
Hillary Clinton is expected to do well in Florida and North Carolina. Her win in OH was a particular relief for her campaign, which grew anxious after Sanders pulled off a surprising win last week in MI.
Trump leads polls ahead of "Super Tuesday 2" for the Republicans while Clinton maintains lead among Democrats.
"Our commander-in-chief has to be able to defend our country, not embarrass it", Clinton said in a speech that largely ignored Sanders. "Guess what? We won in MI ..." What time do Mega Tuesday results come in?
There are 214 delegates up for grabs in Florida, and a win would show he can drive African-Americans and Latinos to the polls.
"There is such a thing as momentum".
Republicans are floating the possibility of a brokered party convention, which occurs when no candidate receives the 1,237 delegates required to win the nomination.
"People say, 'What about the contested convention?" Ryan said in an interview with CNBC.
In Ohio, a victor takes all primary with 66 delegates on the line, the latest poll by CBS - conducted from March 9-11 - shows Trump and Kasich tied at 33 percent each with Ted Cruz surging to 27 percent up from his pre-debate showing of 19 percent. Cruz's father, pastor Rafael Cruz, visited the state just days before those Monday rallies, as well.
All these findings suggest that the March 10th debate may have been a big victor for Cruz.
Florida and OH are the first winner-take-all primaries in the Republican race - meaning they award all of their delegates to the winners of their primaries Tuesday. Florida Sen. Marco Rubio hopes to win his home state, but early predictions weren't looking so good for him. In Iowa, Rubio's delegates will remain his own even if he drops out, according to the University of Virginia's Center for Politics.