First Read is a morning briefing from Meet the Press and the NBC Political Unit on the day's most important political stories and why they matter.

Ohio plays starring role on Separation Tuesday

We’re dubbing today’s presidential primaries -- in Florida, Illinois, Missouri, North Carolina, and Ohio -- Separation Tuesday, because the main storyline is whether or not frontrunners Donald Trump and Hillary Clinton further separate from the rest of the field. As our colleague Peter Alexander put it, tonight could be the difference between a coronation or a contested convention for Donald Trump. For Hillary Clinton, it could be the difference between getting a head start on the general election or becoming mired in another round of expensive primary battles against Bernie Sanders. And for both, the night largely hinges on Ohio. Unless the polls are really wrong, Donald Trump is expected to easily take winner-take-all Florida and its 99 delegates. And he should do well in Illinois, Missouri, and North Carolina, though Ted Cruz has been making a play in all three of those states. But in Ohio, our NBC/WSJ/Marist poll found Trump trailing John Kasich, the state’s governor, by six points. And other recent polling shows Trump either tied or behind in the state. Why is winning both Florida and Ohio important to Trump? Just look at the delegate math:

Trump currently holds an 86-delegate lead over Ted Cruz

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Trump must win 53% of all remaining delegates to reach 1,237 magic number

Cruz must win 80% of all remaining delegates to reach 1,237 magic number

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Trump must win 60% of all remaining delegates to reach 1,237 magic number

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Cruz must win 80% of all remaining delegates to reach 1,237 magic number

Kasich must win 110% of all remaining delegates to reach 1,237 magic number

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Trump must win 70% of all remaining delegates to reach 1,237 magic number

Cruz must win 80% of all remaining delegates to reach 1,237 magic number

Rubio must win 95% of all remaining delegates to reach 1,237 magic number

Kasich must win 110% of all remaining delegates to reach 1,237 magic number

Why Ohio is so important for the Clinton campaign

On the Democratic side, meanwhile, Hillary Clinton tonight is likely to expand her delegate lead over Bernie Sanders (766 to 553 among pledged delegates and 1,198 to 576 among delegates) from just the results of Florida and North Carolina alone. But Ohio will be the difference between a great night and a frustrating night for her. Win Ohio -- and she erases the memory of last week’s Michigan loss, as well as deflate the momentum that Bernie Sanders gained. Lose Ohio -- and it’s Michigan all over again, especially given that every poll out there shows Clinton ahead in the Buckeye State. Look, we know that math is king in this Democratic contest, and Clinton has sizable lead in this proportional system. But Sanders winning Ohio (and thus sweeping the Midwest) would let him hold on the momentum heading into the next month of contests (in Arizona, Idaho, Utah, Alaska, Hawaii, Washington, and Wisconsin), and that could chip away at Clinton’s delegate lead. That’s why Ohio is so big on the Democratic side.

Tonight’s poll closing times: Below are the final poll closings for tonight’s five primaries:

7:30 pm ET: North Carolina, Ohio

8:00 pm ET: Florida, Illinois, Missouri

The last of the establishment GOP frontrunners -- Rubio -- could fall tonight

When this presidential contest started a year ago, and when most of us thought Trump was unlikely to run, we identified three establishment frontrunners: Jeb Bush, Scott Walker, and Marco Rubio. Well, Walker ended his presidential bid two months after formally announcing. Bush, after spending some $82 million over the airwaves throughout the course of the contest, dropped out after finishing fourth in South Carolina. And unless the polling is really off in Florida, Rubio soon could end up exiting the GOP race, though he has been putting on a brave face. “I never said that my campaign is built on the outcome of any specific state,” he said on Fox News last night, per NBC’s Alex Jaffe. And there is a rationale to remain in the GOP race: If Rubio suspends his campaign, many of his delegates would become free agents. So to keep them -- and ensure they don’t go to Trump -- he could stay in the race.

When you’re the momentum candidate and it goes away, there isn’t much else left

But Rubio’s struggles in this campaign weren’t just because he was viewed as part of the GOP establishment. Another reason was that his campaign was built on biography and the appearance of momentum -- and not on the other parts of a successful campaign (like fundraising and sophisticated delegate hunting). Indeed, Rubio’s speech after finishing in third in Iowa epitomizes Rubio’s momentum-is-everything approach. “So this is the moment they said would never happen,” Rubio said, after finishing behind Cruz and Trump in the Hawkeye State. The problem with being the momentum candidate? Once it goes away -- like after Rubio’s rough New Hampshire debate performance -- there isn’t much left. When this campaign started, we said that Rubio would either be Barack Obama ’08 or John Edwards ’04. And unless something extraordinary happens tonight, it’s looking more like the former. Rubio’s problem wasn’t that he attacked Trump personally; his problem was that he didn’t win after building himself up as the momentum candidate.

Trump, Clinton lead in NBC|SurveyMonkey tracking poll

Finally, here’s our latest weekly NBC|SurveyMonkey online tracking poll:

GOP race: Trump 44% (+5), Cruz 24% (+4), Kasich 12% (+3), Rubio 11% (-7)

Dem race: Clinton 54% (-1), Sanders 41% (+3)

Overall, 74% of GOP voters think Trump will win the Republican nomination, and 76% of Democratic voters think Clinton will win the Dem nomination.