If Donald Trump gets 1237 delegates prior to the Republican convention, it’s lights out.

But that is looking less and less likely unless Trump scores a major upset in Wisconsin on Tuesday, at the Colorado convention on April 9 (where the delegates technically are unbound but the sides are fighting to get their people selected as in North Dakota), and wins big in New York on April 19.

Cruz has little chance himself of getting to 1237 prior to the convention, so his strategy is focused on the second ballot.

That strategy, as it is playing out in real time, has two components: Make sure only Trump and Cruz are the choices facing the convention, and make sure there are large defections on the second ballot of delegates pledged to Trump on the first ballot.

Strategy, Part 1: Keep it Trump or Cruz

When it comes to keeping it to two, Trump and Cruz are on the same page. They are planning to fight to keep Kasich from being put in nomination, much less someone not currently in the race.
The NY Times reports:

Senator Ted Cruz of Texas on Monday warned the Republican Party against pushing an “uber-Washington lobbyist” as a possible nominee at a contested convention, insisting that only he or Donald J. Trump should be chosen….

Both Mr. Trump and Mr. Cruz have made the case that Mr. Kasich’s exit from the race would help their campaigns. On Monday, Mr. Cruz invoked a rule from 2012 requiring prospective nominees to have won a majority of delegates in at least eight states, which could be hurdle for Mr. Kasich.

Mr. Cruz said that the rule had been intended to limit the influence of supporters of Ron Paul in 2012, suggesting that party leaders were now receiving a bit of karmic retribution.

“What’s good for the goose is good for the gander,” he said, adding, “We shouldn’t be changing the rules because Washington is unhappy with how the people are voting.”

If someone other than Mr. Cruz or Mr. Trump were to be selected as the nominee, Mr. Cruz said, “the people would quite rightly revolt.”

Cruz didn’t threaten riots or Days of Rage, but he made clear that he wants to give delegates only two choices — Trump or Cruz.

On this point Team Trump is in agreement, as NBC News reports:

The Donald Trump and Ted Cruz campaigns are working to prevent John Kasich from appearing on the ballot at the Republican National Convention in July, MSNBC has learned, an aggressive strategy suggesting the GOP’s leading candidates are girding for a contested convention to select the party’s nominee.

On Sunday, Trump told a supporter that “Kasich shouldn’t be allowed to continue and the RNC shouldn’t allow him to continue.” ….

Both campaigns are backing a rule that would require candidates to achieve a minimum amount of support to get on the ballot, which could block Kasich in Cleveland. That would effectively end his campaign.

So as to Strategy, Part 1 — Cats and Dogs are getting along. But that’s where it ends.

Strategy, Part 2 — Work Prior to the Convention to To Ensure Large-Scale Defections of “Trump Delegates”

Cruz has been fighting hard, using state delegate selection rules, to obtain as many delegates as possible to keep Trump from 1237.

Cruz’s use of Louisiana selection rules netted him more delegates than Trump even though Trump won the popular vote. Trump threatened suit, but as of this writing has not done so.

Cruz also is executing an effective ground game for delegates in North Dakota and in Colorado, where delegates technically are unbound, but Cruz is getting his people selected:

Ted Cruz is emerging as the Republican favorite to win Colorado after he captured the state’s first six national delegates and leads his rivals into this week’s party convention with the most pledged support.

Donald Trump is a distant second place, according to early results, and his supporters are outnumbered among the GOP activists vying for national delegate seats at the Cleveland convention.

But that strategy can only keep Trump from reaching 1237 on the first ballot. The primaries matter at the convention only to the extent a candidate can keep his delegates committed on later ballots, and get another candidate’s delegates to defect.

Since Cruz likely will have significantly fewer first ballot delegates, he needs to get a lot more delegate votes on the second ballot and beyond. Some of those may come from Marco Rubio and John Kasich delegates, but that is unlikely in itself to give Cruz a majority. Cruz needs Trump delegates to defect on ballot 2 and beyond.

Cruz is working to get delegates who technically are pledged to Trump. After the first ballot, they’re free to vote Cruz. The Washington Examiner describes the efforts in Arizona, which Trump won easily:

Sen. Ted Cruz is out-hustling Donald Trump and looks set to ensure many Arizona delegates will defect to him in a convention floor fight.

The Texas senator, who ever since Iowa has played a stealthy ground game in contrast to Trump’s chaotic populism, is taking steps to snatch the Republican presidential nomination from The Donald at the convention in July.

The New York businessman easily won last month’s Arizona primary taking 47 percent to Cruz’s 25 percent, scooping up all 58 of the state’s delegates. That’s nearly 5 percent of the 1,237 Trump needs for the nomination, and they’re tied are to him on the first ballot.

But Cruz, exploiting deep opposition to Trump among grassroots Republicans, has been far more active in Arizona than Trump, insiders say. He’s recruiting candidates for the available 55 delegate slots, that along with the other three delegate positions filled by party leaders, would be allowed to vote for him in a multi-ballot contested convention.

Trump is fighting at the 10,000 foot level, dominating the media and holding huge rallies. It’s worked so far.

But Cruz is undermining Trump by fighting in the trenches with a much more effective ground game at the delegate level.

Trump may find that his delegate lead is a castle built on sand, washed away with the approach of the second convention ballot.

UPDATE 4-5-2016: To Trump supporters complaining that the delegate rules are being used against him, consider how the delegate rules actually have given Trump a huge advantage unrelated to vote numbers. Via David Wasserman of 538 website:

If Ted Cruz wins by a huge margin in Milwaukee’s suburbs, as expected tonight, he’ll get all three delegates from Wisconsin’s 5th Congressional District, which cast 257,017 votes for Mitt Romney in the 2012 general election. But in two weeks, Donald Trump could capture just as many delegates by winning a majority of the vote in New York’s heavily Latino, Bronx-based 15th Congressional District, which cast only 5,315 votes for Romney four years ago.

Three weeks ago, Trump won three times as many delegates — nine — at the Northern Mariana Islands convention, which drew just 471 participants.

Welcome to Trump’s “rotten boroughs,” the curious places where mere handfuls of voters (relatively speaking) are keeping him in the hunt for the 1,237 delegates required to clinch the GOP nomination. And the quirks of the Republican National Committee’s delegate math — the ones once considered a safeguard against an upsetting of the party order by an insurgent like Trump — will take on a more pronounced role as the GOP contest enters its late stages.