March 15 has long looked like the most pivotal date on the GOP primary calendar. And although Florida and Ohio are hogging the spotlight because they are the sites of Marco Rubio and John Kasich’s “last stands,” don’t forget that two other states could help Donald Trump become essentially unstoppable in his quest for the nomination: Illinois and Missouri.

Sure, far more delegates were at stake on Super Tuesday (595) than will be awarded March 15 (367), but the Super Tuesday delegates were all awarded proportionally. By the Republican National Committee’s rules, March 15 is the kickoff of the high-stakes winner-take-all season. Of the five states voting Tuesday — Florida, Illinois, Missouri, North Carolina and Ohio — only North Carolina will truly award its delegates on a proportional basis, making it the least valuable prize of the night.

Florida and Ohio are hyped up not only because of their favorite sons (for good reason), but also because they are the only “true” winner-take-all states: The Florida winner will claim 99 delegates, and the Ohio winner will claim 66 — no ifs, ands or buts. However, Illinois and Missouri could functionally become winner-take-all too. Both states award their delegates on a congressional-district level. So if Trump (or someone else) sweeps their congressional districts, that candidate will win all the delegates. Together, Missouri and Illinois will award 121 delegates — which would go a long way in helping Trump stay “on track” for the nomination even if he loses either Florida or Ohio.

And so Illinois and Missouri are a big part of the reason Tuesday is such a huge fork in the road. If Trump sweeps Florida, Illinois, Missouri and Ohio, he will have at least 748 delegates and would need to win only 44 percent of all remaining delegates, a remarkably low bar, potentially ending the nomination fight.

If Trump loses Ohio but still wins Florida and sweeps Illinois and Missouri, he would need to win 50 percent of all other remaining delegates, a slightly higher bar but still very doable — and he would probably still be “on pace” for the nomination according to our delegate targets. But if Trump were to lose both Ohio and Florida, along with, let’s say, half of Illinois’s and Missouri’s districts, he could find himself needing to win 63 percent of remaining delegates to clinch the nomination, a much less plausible goal, considerably raising the odds of a contested convention in Cleveland.

Which of those scenarios is most likely? There haven’t been any polls of Missouri since August, but considering the state’s relatively large share of evangelicals (36 percent of the population), it would seem that Ted Cruz would be Trump’s main threat in the Show-Me State. But, unlike the last three states Cruz has won (Kansas, Maine and Idaho), Missouri has an open primary — the type of contest Trump has dominated thus far. In fact, Illinois and Missouri are among the relatively few states remaining to vote where the rules permit non-Republicans to vote in the party primary.

Technically, Illinois holds a “loophole” primary in which district-level delegates will be elected directly on the ballot. But unlike in the loophole primary in Pennsylvania, delegates’ presidential preferences are stated on the ballot, making it highly likely that the preferred presidential candidate will win all three delegates at stake in a given congressional district. The only recent poll taken in Illinois, a Chicago Tribune survey from the first week of March, showed Trump leading with 32 percent, to 22 percent for Cruz, 21 percent for Rubio and 18 percent for Kasich. A double-digit lead raises the possibility that Trump could sweep most or all of the 18 congressional districts.

Trump currently holds 462 delegates, 43 percent of the 1,065 delegates that have been at stake so far. But up until now, just 5 percent of all GOP delegates (the 50 Trump won in South Carolina) have been awarded on a winner-take-all basis. From March 15 forward, a whopping 64 percent of delegates will be awarded on a winner-take-all basis (39 percent based on statewide winners plus 25 percent based on district-level winners). This means continued Trump pluralities would be more than sufficient to earn him a majority of the 2,472 delegates by June.