The Republican nightmare scenario is an actual possibility if something big doesn't change fast: Donald Trump could all but ensure he'll get more delegates than any one of his rivals by March 15.

Trump has already won three of the first four states, and remains at the top of polls both nationally and in most of the next states to vote. And now an incredibly important phase of the contest is about to begin. In the first half of March, 24 states will hold primaries and caucuses. When the dust settles on March 15, about 58 percent of delegates will already be bound to one candidate or another.

Now, technically Trump almost certainly won't have clinched the nomination by then — he needs just over half of total delegates to do that, and most of the early March delegates are given out proportionally. But he could build up such a huge lead by this point that it would be tremendously difficult for any one candidate to catch him unless there's some sudden and remarkably dramatic change in the race.

For instance, if Trump racks up a 300-delegate lead over Marco Rubio by March 15 — a crude estimate of what would happen if he, Rubio, and Ted Cruz all stay in the race and match their average delegate hauls in proportional states so far — Rubio would need to win nearly two-thirds of the remaining delegates to catch up to him afterward. That's a really implausible amount.

The speed of the upcoming contests is terrible for GOP elites, because their new favorite theory about how to stop Trump banks on time. They want the field of candidates to winnow, leaving just Rubio, who they think can defeat Trump one on one. But the longer the field remains divided, the bigger the lead Trump is likely to rack up in the next thee weeks, when so many delegates are allotted. Indeed, this head-to-head matchup might not actually happen until more than half the delegates are gone.

The worst-case scenario for the party is if Trump's winning streak culminates with him beating Rubio in the senator's home state of Florida on March 15. And it's not all that implausible — Trump has led every poll of the state this year by double digits. Not only would this be a devastating humiliation for Rubio, it would be a massive delegate haul for Trump, since Florida is a winner-take-all state. So even though Trump would need some more delegates afterward to get over the top, if that scenario comes about, we could well look back on March 15 as the date the nomination was truly decided.

But even if Rubio does win Florida, Trump could amass such a delegate lead that it's all but impossible for any one of his rivals to catch up. In that case, elites may well basically abandon hope of getting Rubio the delegate majority — and instead focus on ensuring that Trump doesn't get the majority of delegates he needs, so his nomination could be blocked through a contested convention. But denying the nomination to the clear choice of a plurality of the party's voters could be a PR disaster akin to the infamous Democratic convention of 1968.

More than half of delegates will be gone by mid-March

Now, there's no reason to take for granted that Rubio would even beat Trump in a one-on-one matchup. For instance, Rubio could well wilt under Trump's inevitable negative attacks, or Trump could pick up a good share of supporters from other candidates who might drop out (like Cruz).

But let's table those concerns for now. The biggest question GOP elites should have at the moment is whether the matchup will materialize before Trump racks up a gigantic delegate lead.

Because things are about to start moving very quickly indeed — in the next 21 days, a bit more than half of all delegates in the contest will be handed out to candidates:

Super Tuesday on March 1 is the first big multistate contest. This year, it's been dubbed the "SEC primary" because so many Southern states are scheduled for this day. Texas, Georgia, Tennessee, Alabama, Virginia, Oklahoma, Massachusetts, Arkansas, Minnesota, Alaska, and Vermont — which together account for a quarter of Republican delegates — will all hold primaries or caucuses then.

is the first big multistate contest. This year, it's been dubbed the "SEC primary" because so many Southern states are scheduled for this day. Texas, Georgia, Tennessee, Alabama, Virginia, Oklahoma, Massachusetts, Arkansas, Minnesota, Alaska, and Vermont — which together account for a quarter of Republican delegates — will all hold primaries or caucuses then. Then, between March 5 and March 12 , eight more states, the District of Columbia, Puerto Rico, and Guam will all hold their contests.

, eight more states, the District of Columbia, Puerto Rico, and Guam will all hold their contests. And then we hit Super Tuesday II on March 15 , in which Florida, North Carolina, Illinois, Ohio, Missouri, and the Northern Mariana Islands will all vote. By the time the dust settles, about 58 percent of delegates will be allotted.

, in which Florida, North Carolina, Illinois, Ohio, Missouri, and the Northern Mariana Islands will all vote. By the time the dust settles, about 58 percent of delegates will be allotted. After the Ides of March, things slow down. Over a little less than three months, 37 percent of the remaining delegates will be allocated in primaries and caucuses, culminating on June 7.

The other 5 percent or so of delegates will be determined through more arcane methods in a few states and territories that have chosen not to use traditional primaries and caucuses. Some of them won't be bound to any candidate.

So this idea that there's still ample time for a Rubio/Trump matchup to materialize may be quite mistaken. Because so many contests are packed into the first half of March, the GOP nomination contest really will be just about three-fifths of the way finished by March 15 — less than three weeks from now — if you go by the delegate count.

Kasich's continued presence in the race is a problem for Rubio

In addition to this frontloading problem, the specific lineup of states set to vote on Super Tuesday — mainly Southern ones — perversely gives both John Kasich and Ted Cruz separate excuses to stay in the race longer, even though it looks increasingly unlikely that either can win.

Kasich, who is now seen as Rubio's sole remaining rival for the support of more mainstream Republican voters, is basically arguing that Super Tuesday doesn't count for him because it's too Southern — and that he intends to make his stand in more favorable territory.

First, he'll try to pick up some delegates from Northeastern states like Massachusetts and Vermont on Super Tuesday.

Then he hopes his next big win will be in Michigan on March 8, followed by a victory in his winner-take-all home state of Ohio on March 15, and perhaps in nearby Illinois as well.

Sounds nice — but recent polls of Massachusetts, Vermont, Michigan, and Illinois have all shown Trump far out in front, with Rubio and Kasich effectively tied for a weak second (sometimes alongside Cruz). So by staying in, Kasich is likely preventing Rubio from winning a stronger second place in these states. And since they allot delegates proportionally, Rubio's early March delegate haul will be hurt as a result.

So Kasich will surely face immense pressure to drop out — indeed, he already is. But from Kasich's perspective, it makes perfect sense that he's staying in. Sure, he hasn't yet won a state, but neither has Rubio. And Kasich doesn't have that many fewer delegates than Rubio. Why quit when so little has been decided?

But in the contests so far, Kasich finished terribly — in the single digits — in Iowa, South Carolina, and Nevada, which seems to reveal that he has little appeal outside deep blue states. Rubio, however, has hit double digits in every state so far, and topped 20 percent support in all but New Hampshire. The point is that Rubio seems to have broader appeal but that Kasich is, for the time being at least, holding him back.

Cruz could hurt Rubio's Super Tuesday delegate haul

Besides Ben Carson, whose support is dwindling, the other obstacle to the Trump-Rubio showdown is Ted Cruz.

Unlike Rubio, Cruz has won a state. But his path to the nomination always entailed doing well in the South, and if Trump outperforms him there as he did in South Carolina, he's toast. This is particularly urgent because nearly every Southern state will have already voted by the time March ends, with the remaining delegate prizes being located in less Cruz-friendly regions.

It's not intuitively clear whether Cruz is drawing more voters from Trump or Rubio — he is staunchly anti-immigration reform, like Trump, but he is also an actual conservative, like Rubio. However, a recent NBC/SurveyMonkey poll does suggest Rubio would benefit somewhat more from Cruz's exit.

But Cruz clearly isn't going anywhere at least until that batch of Southern states votes on Super Tuesday. And if he performs well in his home state of Texas, the biggest state to vote that day, he'll win a lot of delegates and likely stay in even longer.

Furthermore, the four biggest states to vote on Super Tuesday — Texas, Georgia, Tennessee, and Alabama — theoretically allot delegates proportionally. But, though their details vary, they all have two broad rules that could be very bad for Rubio if Cruz and Trump both keep running strong.

First there's the 20 percent threshold . A candidate must get 20 percent or more of the vote statewide to qualify for delegates. Someone who falls below that threshold statewide gets none of the delegates being allotted statewide.

. A candidate must get 20 percent or more of the vote statewide to qualify for delegates. Someone who falls below that threshold statewide gets none of the delegates being allotted statewide. Then there's the 2:1 congressional district rule. In addition to those statewide delegates, these states also allot three delegates "proportionally" according to the results in each of their many congressional districts. But there's little true proportionality to be found — in each district, two delegates go to the winner, one goes to the runner-up, and anyone finishing third or below gets nothing.

Now, in South Carolina, Rubio finished with 22.5 percent of the vote, putting him in second place. If he can duplicate or better that all across those big Southern Super Tuesday states, topping Cruz, neither of those two rules would be a problem for him. And indeed, a poll this week shows Rubio, not Cruz, in second in Georgia.

But if Cruz manages to claw his way back into second, Rubio risks falling afoul of those two rules and having his delegate haul in the four biggest Super Tuesday states sharply reduced. This will make him look weaker overall, and the weaker he looks, the less incentive his rivals will have to quit and the harder it will be for him to catch up to Trump.

These are risks to Rubio. However, he still might prefer this to a true one-on-one showdown with Trump in the South — because Texas, Georgia, Alabama, and a few other states have winner-take-all triggers. That is, if one candidate tops 50 percent statewide, he gets all the statewide delegates (though congressional district delegates are still broken out separately). Topping 50 percent is far easier when there are fewer candidates in the race, and it could well be Cruz's presence in the race that stops Trump from doing so.

Trump will keep winning more delegates after March 15

To some extent, it's common sense that the bigger the delegate lead Trump accumulates by March 15, the tougher it will be for Rubio to catch up. But the specifics of the calendar could make things even worse.

Though the contest will stretch on for nearly three more months, only 37 percent of the remaining delegates will still be up for grabs in primaries and caucuses. Furthermore:

A third of these delegates are from the Northeast — Trump's home region, where he has run strongly so far among Republicans. These include his home state of New York and nearby New Jersey.

Theoretically, "winner-take-all" states that allot all their delegates to the statewide victor could help Rubio make up some ground. Yet only seven states voting in this period are true winner-take-all states, allotting all their delegates to the statewide victor. The biggest of them is Arizona, which is not all that big.

Of the other states, some award delegates to winners of congressional districts in addition to the statewide winner, and some allot delegates proportionally. But since the other contenders will likely be gone after this point, it will be easy for Trump to pick up lots of delegates even in the states he loses.

Again, that's not to say Rubio can't beat Trump in this fashion. But the bigger a lead Trump amasses by March 15, the tougher it will be for Rubio to pull off.

Rubio needs to start winning states

The best hope Rubio has is that Republican voters everywhere decide to make the winnowing process irrelevant — by concluding that Cruz and Kasich are finished and that the race is down to Rubio and Trump.

When I spoke about delegate math with Josh Putnam, a University of Georgia lecturer and expert on nominating rules who runs the Frontloading HQ website, he brought up the concept of "zombie candidates" — people who are technically still in the race but are effectively done, with their previous supporters deserting them for more viable options.

That could well happen. Many voters are strategic. They don't want to throw away their vote on someone they think can't win. If they keep hearing Rubio is the guy, then they might conclude that, well, Rubio's the guy.

But Rubio has a serious problem in making this case: He hasn't yet won a state.

Mark Mellman, a leading Democratic pollster, told me last year that the early-state results can give candidates visibility and viability. Visibility is obvious — a strong performance in an early state helps voters learn who you are. As for viability, "There’s no better evidence that you can win than having won," Mellman said.

Rubio hasn't won, though. So voters who are politically savvy may have gamed out the primary calendar and delegate math and realized that Cruz and Kasich are in deep trouble. But to the broader electorate, Rubio could have a hard time making his case that he's the only non-Trump candidate left until he manages to put some points on the board. We'll see if he does.