Even though Tuesday may not have more delegates or states in play than Super Tuesday, March 1, had, it's still a big day, with more than 1,000 delegates at stake. More importantly, the results could end up deciding who the Republican and Democratic presidential candidates will be.
Five states are casting votes on March 15, along with one U.S. territory on the GOP side.
For Donald Trump's Republican rivals, it could be their last chance to stop his march toward the nomination, as the first winner-take-all states begin to vote. Florida Sen. Marco Rubio is trying to topple Trump in Rubio's home state; a loss would likely prove fatal for Rubio's campaign. Ohio Gov. John Kasich is trying to fend Trump off on his home turf, with seemingly more success. Meanwhile, Texas Sen. Ted Cruz hopes he can grow his share of delegates and continue to make the argument he's the only candidate who can catch Trump.
On the Democratic side, Vermont Sen. Bernie Sanders hopes his surprise win last week in Michigan means he can make inroads with other Rust Belt voters in Ohio, Illinois and elsewhere. Former Secretary of State Hillary Clinton would like to blunt Sanders' newfound momentum and notch wins in the Midwest.
Coming close no longer cuts it on Tuesday, at least for Republicans. The biggest prizes of the night, Florida and Ohio, are winner-take-all contests. For Republicans in the rest of the states and Democrats in all their contests, delegates will still be awarded proportionally.
Here's the state of play and what to watch in each state, ordered by poll closing time.
Caucuses began at 4 a.m. ET on Tuesday (6 p.m. local time)
Northern Mariana Islands
Trump began the big day of voting with a win in the U.S. commonwealth's GOP caucuses. He took 73 percent of the vote (343 votes out of just 471 cast), which will give Trump all nine available delegates. Cruz was second with 113 votes, while Kasich got 10 votes and Rubio took just 5
Polls close at 7:30 p.m. ET
The Buckeye State is the first winner-take-all contest of the night on the GOP side, and it might be Republicans' best chance to deny Trump a win. Recent polls have shown Kasich with a small but steady lead over Trump, and he was hoping to shore up support in the final hours by campaigning with 2012 GOP nominee Mitt Romney on Monday. Even his rivals have seemed to acknowledge voting for Kasich is a strategic pivot in the #NeverTrump movement — last week Rubio's campaign encouraged his supporters to vote for Kasich in the Buckeye State. Early returns in the state could give a good indication of whether Tuesday will be a sweep for Trump or if there are chinks in his armor.
Polls have also shown a tightening contest on the Democratic side between Sanders and Clinton. She won the state in 2008 over Obama, but the Vermont senator is hoping to build on the constituencies that helped him deliver a surprise Michigan win a week ago — blue-collar voters, particularly white men upset with trade policies and the economy, as NPR's Tamara Keith explored. Another thing that could boost Sanders: Last week a judge ruled that 17-year-olds could vote in the primary, a boon to the candidate who has done better with younger voters.
Polls show Trump with a double-digit lead over Cruz in the Tar Heel State, with both Rubio and Kasich in the distance. Cruz needs to make inroads in the more rural, conservative areas of the state, but if Trump cleans up there and wins metro areas, he'll be hard to topple. North Carolina's 72 delegates — the second-most available for the GOP on Tuesday — will be awarded proportionally, and there's no threshold to qualify.
Clinton appears to have a comfortable lead on the Democratic side, continuing her strength in Southern states that have significant African-American populations. Still, her margin probably won't be as big as the 2-to-1 wins she enjoyed in South Carolina and Alabama, for example.
8 p.m. ET
This is do-or-die for Rubio. The GOP hopeful has promised in the past he'll win his home state of Florida, pointing to the winner-take-all prize as where his campaign can be revitalized after far less significant wins in the Minnesota caucuses and in Puerto Rico and the District of Columbia. But while he once trailed Trump by only single digits, the latest surveys from the state now show the state's sitting senator could lose by a 2-to-1 margin to Trump — and Cruz is hot on his heels in third. If Rubio doesn't get one single delegate from the state that launched him to GOP stardom just six years ago, his campaign is effectively over. Now, he must hope the polls are radically wrong, like they were for Sanders a week ago in Michigan.
Clinton also has a comfortable lead in Florida polls, and the Sunshine State (and its 214 pledged delegates, which will be awarded proportionally) could end up being the bright spot of the night for her even if she's in a fight with Sanders in the Midwest.
The Land of Lincoln could end up being one of the closest contests of the night for both parties. Trump has a narrow lead in Illinois GOP polling, but Cruz has made a late push and spent all day Monday barnstorming the state as surveys showed the race tightening. Illinois isn't a winner-take-all contest, but instead awards 15 of its 69 delegates to the statewide winner and the rest based on a formula by congressional district wins. So Cruz's strategy to pay attention to more far-flung corners of the state could pay dividends.
Clinton's lead over Sanders has evaporated in the state; it's now a neck-and-neck contest. Like in Ohio, Sanders needs to perform well with working-class, white voters and has been hoping his hits on Clinton over trade and economic inequality can have special resonance in the manufacturing-heavy parts of the state.
There's been virtually no polling on the GOP side in the state, so it's hard to handicap the race. Trump held several raucous rallies here over the weekend, while Cruz has argued he's closed the gap in the Show Me State and can stop Trump here. The Texas senator's campaign manager, Jeff Roe, is a longtime strategist in the state and knows the ins and outs of how to win close races. This could be one of the most interesting contests to watch as the returns trickle in Tuesday night, and an important place where Cruz can eat into Trump's delegate lead.
Scant surveys have also shown a tight race on the Democratic side, where Sanders is hoping for another Midwest upset. It has the smallest number of delegates up for grabs on Tuesday, with just 71 pledged delegates, which will be distributed proportionally.