There's both a technical and a symbolic reason for Ohio's status as a first among equals in today's voting.
Let's start on the numbers side. Both Florida (99 delegates) and Ohio (66 delegates) are winner-take-all states, meaning that whoever wins the state gets all of those delegates. Trump is near-certain to win Florida. That's 99 more delegates. He's also ahead in polling in North Carolina, Missouri and Illinois — all of which dole out delegates proportionally. (More on that below.) That makes Ohio absolutely critical for everyone not named Trump who is hoping to keep him under the 1,237 delegates he needs to formally claim the Republican nomination.
Here's what Trump's delegate totals look like with — and without — an Ohio win.
Winning Ohio gives Trump a 300-delegate vote lead over Cruz and would give him approximately 60 percent of the delegates he needs to officially be the nominee. (Trump currently has an 86-delegate lead over Cruz.)
And now for a little bit more math. If Trump wins Florida and Ohio, he would need to win only 52 percent of the remaining delegates to be the nominee, according to calculations made by NBC News. That's a higher rate than he's won them to date but the geography of the remaining calendar — lots of coastal states — clearly favors Trump over, say, Cruz. If, on the other hand, Trump wins Florida but loses Ohio, he needs to win roughly 60 percent of the remaining delegates. That's still doable but a significantly higher lift.
(Editor's note, 2:30 pm: As the Cook Political Report's delegate expert David Wasserman helpfully notes, describing Illinois and Missouri as proportional allocation of delegates is a bit of a misnomer by me. In Illinois, the winner of the statewide race gets 12 delegates; the winner in each of the state's 18 congressional districts gets three delegates per district won. In Missouri, the statewide winner gets 12 delegates while, if no one gets 50 percent statewide, the remainder of the delegates are doled out to the winner of each of the state's eight congressional districts.)
Now, to the symbolic argument.
Ohio and Florida have been the two most important states in general election presidential races since at least 2000. They are big. They are diverse. They tend to determine who wins the White House and who, well, doesn't.
If Trump sweeps both today and in the process beats hometown favorites in each, it's very hard to make a compelling argument that Republican voters aren't making clear what their preference is in the race. He has already won in the west, in the northeast and in the south. He has won 15 of the 27 contests — including a victory already today in the Northern Marianas Islands — that have voted thus far in the primary process. If he wins all five states on Tuesday — including Ohio and Florida — that number jumps to 20 out of 32 (62.5 percent of all contests).
Yes, as I noted above, Trump still needs to win a majority of the delegates going forward even if he takes Florida and Ohio. And yes, this is all a math problem in the end. But momentum can, at times, Trump — ahem — math. And denying Donald Trump the nomination if he sweeps Ohio and Florida (as well as Illinois, North Carolina and Missouri) becomes extremely difficult.
For the anti-Trump movement, it's pretty clear that it's Ohio or bust. We'll know after tonight whether Trump can be slowed or even, possibly, stopped.