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.

What Tuesday’s Wisconsin primary will tell us (and what it won’t)

Ahead of tomorrow’s Wisconsin primary, for both the Republicans and Democrats, it’s worth noting what Tuesday’s results will tell us -- and what they won’t. What they will tell us:

If the “Stop Trump” forces can finally notch a big win: Since Mitt Romney delivered his anti-Trump speech a month ago, the “Stop Trump” forces haven’t been able to beat the real-estate mogul -- outside of Ohio (John Kasich’s home state), Idaho, and Utah. So a win in Wisconsin would be a real victory for the #NeverTrump folks.

If Ted Cruz can narrow his delegate deficit with Trump: Wisconsin awards 42 delegates, and it’s possible that Cruz can win all of them -- given that they’re allocated winner-take-all by both statewide vote (15 total delegates) and results in individual congressional districts (three delegates in each of the state’s eight districts). So Trump’s current lead could shrink from 275 delegates to 233 (if Cruz wins all of the delegates). Or it could shrink to around 260 (if Trump is able to win some of the congressional districts). And in race where EVERY delegate matters, this is important.

If Bernie Sanders can truly cut into Hillary Clinton’s delegate lead: Unlike the GOP side, all Democratic delegates are allocated on a proportional basis, which means a two- or four-point Sanders win (as the current polling reflects) won’t really alter the math in the Democratic race. If Sanders wants to overtake Clinton in pledged delegates, he needs to win in Wisconsin (as well as New York and California) by MUCH bigger margins.

What tomorrow’s Wisconsin primary won’t tell us

If a loss dooms Trump’s path to the magic number of 1,237 delegates: Wisconsin awards 42 delegates on the Republican side. But New York (on April 19) gives out 95, and Connecticut/Delaware/Maryland/Pennsylvania/Rhode Island (April 26) offer 172, which means that Trump can more than make up for a Wisconsin loss as the GOP contest turns to the Northeast.

If Cruz really has the momentum: Similarly, while a Cruz win tomorrow would certainly boost his campaign, it wouldn’t tell us if the race has fundamentally changed. For that to happen, Cruz would need to overperform in the Northeast.

If Clinton is in real trouble: Make no mistake: A Sanders win in Wisconsin would launch another round of tough stories for Hillary Clinton and her campaign. But for Team Clinton to truly be in trouble -- and for the Democratic alarm bell to sound off -- Sanders has to beat her in New York in two weeks.

Is the Sanders campaign all but admitting defeat?

The question is whether this kind of story -- the Sanders campaign all but admitting to the New York Times that Sanders is unlikely to beat Clinton -- ends up hurting him in Wisconsin and New York. “Mr. Sanders is now campaigning more effectively than many expected, exposing Mrs. Clinton’s weaknesses as a candidate, and is positioning himself to win contests like the Wisconsin primary on Tuesday. But allies and advisers of Mr. Sanders say they missed opportunities to run an aggressive political operation in 2015 that would have presented more of a challenge to Mrs. Clinton. She has now firmly built a big lead in delegates needed to clinch the nomination — a margin that would be smaller if Mr. Sanders had run differently last year, according to interviews with more than 15 people who are on his team or close to him.” It’s hard to sustain a revolution when the revolutionaries are admitting that the war is over and that they lost. And that’s the impression you get after reading the story. Will this affect Sanders’ voter mobilization, his grassroots donor base, and his attacks on Clinton on the campaign trail?

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An important reminder: Delegates -- not the voters -- select the presidential nominee

On “Meet the Press” yesterday, RNC Chair Reince Priebus made an important acknowledgement: The people who choose the GOP presidential nominee aren’t the voters – but rather the delegates who will be headed to Cleveland in July. “The nomination is won on the floor in Cleveland by the majority of delegates that get empowered and by the vote of the people,” Priebus said. “So the voters in these states, their votes end up causing delegates to be bound to candidates, and those delegates will have to vote for those candidates on the floor.” And in that delegate race, there were some developments over the weekend: “Ted Cruz’s preferred candidates won the vast majority of convention delegates available in North Dakota over the weekend, taking 18 of 25 slots in the state in another show of organizational strength over Donald Trump,” Politico writes. But NBC’s Decision Desk gives Cruz seven delegates to Trump’s 1, with the other 20 uncommitted. And in Tennessee, NBC’s Alex Jaffe and Vaughn Hillyard report, the state party filled Trump’s delegates with non-Trump people. Priebus responded to that on “Meet” yesterday. “Donald Trump has 33 delegates in Tennessee. And they're bound to Donald Trump, not just for one vote, but in Tennessee, they're actually bound for two votes. And nothing can change that.” But here’s the rub: What happens on a third vote? It’s another reminder that if the GOP nomination goes to a second (or third) vote, Trump probably won’t be your 2016 GOP nominee.

Trump, Cruz forces team up to block Kasich from Cleveland

MSNBC’s Ari Melber has more GOP convention/delegate intrigue. “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… 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. Kasich cannot mathematically win the nomination through the primaries, so his only hope is to win on a convention ballot.”

Sanders flips two delegates in Nevada (for now)

When it comes to the delegate race on the Democratic side, here’s Nevada political reporter Jon Ralston: “Despite losing the state on Feb. 20 in the caucus, Bernie Sanders' campaign swarmed the Clark County caucus and probably flipped two delegates from Hillary Clinton's camp... That is expected to switch two delegates to Sanders, giving Clinton an 18-17 lead in Nevada.” But per NBC's Decision Desk, it is critical to remember that the At-Large/PLEO numbers can and probably will change at the Nevada state convention on May 14-16. These county convention results do NOT change the winner of the Nevada caucuses. Clinton won Nevada, based on the Feb. 20 results. The results of the succeeding levels do not change that.

Where the Democratic delegate race currently stands

Here are NBC’s updated delegate numbers after accounting for the two flipped delegates in Nevada.

In pledged delegates, Clinton holds a 255-delegate lead over Sanders

Clinton 1235 (56% of delegates won)

Sanders 980 (44%)

In overall delegates (including superdelegates), Clinton holds a 680-delegate lead over Sanders

Clinton 1692 (63%)

Sanders 1012 (37%)

Clinton must win 34% of remaining delegates to hit 2383 magic number

Sanders must win 66% of remaining delegates to hit 2383 magic number

Where the GOP delegate race currently stands

Trump has a 275-delegate lead over Cruz

Trump 750 (47% of delegates won)

Cruz 475 (30%)

Rubio 172 (11%)

Kasich 143 (9%)

Trump must win 56% of remaining delegates to hit 1,237 magic number

Cruz must win 88% of remaining delegates to hit 1,237 magic number

Kasich must win 126% of remaining delegates to hit 1,237 magic number

On the trail

Hillary Clinton campaigns in New York, while husband Bill stumps in Wisconsin… Bernie Sanders also spends his day in Wisconsin… As does Donald Trump, who hits La Crosse and Superior… Ted Cruz campaigns in Waukesha, WI and Madison, WI… And John Kasich is in New York.