Update: Donald Trump has won the New York primary, according to calls by multiple media outlets. The Democratic race has not yet been called.
Original story: For the past few weeks, both parties' presidential frontrunners have appeared to be on the ropes. Hillary Clinton has lost seven of the past eight contests to Bernie Sanders and seen her national primary polling lead shrink to nearly nothing. Meanwhile, Donald Trump lost badly to Ted Cruz in the Wisconsin primary and has been utterly bungling the delegate selection game.
But if the polls are right, Clinton and Trump are set to turn things around today, as voting moves to both candidates' home turf in the New York primary. Polls close at 9 pm Eastern.
Trump has been utterly blowing out his competition in polls, and is currently leading by 30 points in the RealClearPolitics average. And Cruz, who famously disparaged "New York values" in a January debate, is far back in third place, behind even John Kasich.
Clinton's lead is smaller than Trump's, but still sizable — she's ahead of Sanders by about 12 points in recent polls. However, this does represent an improvement for Sanders from late March, when he trailed her by the far greater margin of 30 points.
The true import of today's primary, though, will lie in what it means for each race's delegate count. New York is a populous state with a great number of delegates in play — indeed, on both sides it's the biggest delegate haul remaining except for California on June 7. Yet, as usual, the parties' rules for allotting those delegates are quite different.
What to watch on the Republican side: How often will Trump top 50 percent?
Barring some enormous polling error, Donald Trump has this race in the bag. There's still a great deal of drama, though, and it's related to just how much closer he'll manage to get to a delegate majority.
Trump's path to a majority basically requires that he do very well indeed in New York (and in the other Northeastern and mid-Atlantic states voting next week). An ordinary win would be a disappointment — he needs to win the vast majority of New York's delegates, probably above 80.
So the magic number here for Trump is 50 percent +1 — both statewide, and in each of New York's 27 congressional districts. That's because the state allots its delegates proportionally unless one candidate wins an outright majority, in which case that whole batch of delegates is given to him or her.
Since only 14 of New York's 95 delegates are allotted based on the statewide vote, most of the action will be happening at the congressional district level. If Trump tops 50 percent in a district, he'll win all three of that district's delegates. If he wins the district but doesn't reach 50 percent, he'll get two of three. (If he comes in second he'll get just one.)
There's a great deal of geographic and demographic variation in New York's districts — Harry Enten has a great rundown at FiveThirtyEight. But if Trump's strength is overwhelming in certain parts of the state like Long Island, and weaker in others like Manhattan and Brooklyn, he could well end up missing his delegate target.
This is also a state where John Kasich's continued presence in the race could hurt Trump badly. If New York voters had to choose between just Trump and Cruz, Trump would be highly likely to top that 50 percent winner-take-all threshold in every district he wins. Having three candidates in the race makes it tougher to reach 50 percent, though, even considering Cruz's deep unpopularity in the state.
What to watch on the Democratic side: Will Clinton pad her delegate lead?
There has been enough bad news for Clinton of late — her unimpressive general election polling, her shrinking lead among national Democrats, and her recent losing streak — that even a narrow win could set off alarm bells among her backers, considering that just last month she led the state by 30 points in polls.
But in the delegate count, any win is good for her at this point. Currently, Clinton is about 200 pledged delegates ahead of Sanders. We're already far enough along that it's quite difficult for Sanders to catch up with her — he needs to win more than 56 percent of the remaining pledged delegates in order to do that. So if Clinton wins New York as the polls predict, that gap will grow even wider.
Now, every Democratic contest has to allocate its delegates proportionally. Usually that means that if Hillary Clinton gets 55 percent of the votes, she'll get about 55 percent of the delegates.
Yet New York doesn't just tally the statewide result — delegates are awarded proportionally based on the results in all 27 of New York's congressional districts too, as the Green Papers explains.
Out of the state's 247 pledged delegates, 84 will be awarded based on the statewide result, and the other 163 will be awarded in congressional districts (five to seven per district). So it may take some time to determine exactly how big a delegate lead the winner comes away with.
Another thing to keep an eye on is that New York is a closed primary — only registered Democrats can vote in the Democratic race. Furthermore, New York makes it unusually difficult for a registered voter to switch his or her party registration — the deadline to do so was last October. This could all be bad news for Sanders, who has tended to perform strongly among independents.