Frontrunners Hillary Clinton and Donald Trump have long held the upper hand in their respective primaries — and today's elections present each a big opportunity to jump even further ahead in the delegate count.
Five states are going to the polls, and they're all in the Northeast or mid-Atlantic: Pennsylvania, Maryland, Connecticut, Rhode Island, and Delaware. Polls close for all five states at 8 pm Eastern.
For both parties, this will be the biggest single-day delegate haul until June 7, the last major day of voting. Accordingly, it's the biggest chance the underdogs have to shake up the math before then.
But if the polls are right, that won't happen — and instead, Clinton and Trump will expand their leads even further. The two are ahead in the polling averages for all five states voting today.
For Democrats, Sanders is at least close behind in Rhode Island and Connecticut, trailing Clinton by single digits. But things look grimmer for him in the bigger states of Maryland and Pennsylvania. If things shake out this way, his already very difficult delegate math — he needs 58 percent of remaining pledged delegates to get a pledged delegate majority at this point, while Clinton only needs 41 percent — will grow even more daunting.
As for Republicans, Trump appears to be remarkably strong in the Northeast — neither Ted Cruz or John Kasich is within 20 points of him in the polling average for any state voting today, so Trump's the favorite to win all five states.
Things are never so simple with Republican delegates, though. Trump is shooting for that outright majority of 1,237 delegates that he needs to avoid a contested convention. To get there, he'll need to do more than just win all five states today — he'll have to rack up as many of today's 172 delegates as possible by meeting various requirements.
What to watch in tonight's results for Donald Trump
To maximize his delegate haul today, Trump has to do something different in each state.
Things are simplest in Delaware — Trump gets all 16 delegates in the state if he wins, by any amount.
But in Maryland, Trump needs to win both statewide (for 14 delegates), and in each of the state's eight congressional districts (which combine for 24 delegates).
Then in Connecticut, Trump needs to win statewide with more than 50 percent of the vote (for 13 delegates), and to win in each of the state's five congressional districts (which combine for 15 delegates).
Rhode Island uses a proportional system, so Trump can't win all 19 of its delegates — but the higher his final vote total, the more he'll get. (Also, if Ted Cruz falls below 10 percent of the vote there — a threshold polls show him near — he'll get zero and Trump will win more delegates accordingly.)
Finally, there's Pennsylvania. The statewide winner gets 17 delegates, which is easy enough. But for the other 54 delegates — who are elected directly on the ballot and unbound — things get complicated.
Pennsylvania has a really weird system
Pennsylvania Republicans use one of the most complex systems of any state in the primary season — and that's saying something. They list the names of various delegate candidates on primary ballots, so voters can choose their delegates directly.
Overall, there are 162 delegate candidates listed on the ballot running for 54 slots — three slots per district. (In two districts, the 1st and 14th, only three candidates successfully filed, meaning they win the slots by default unless they lose to a write-in campaign.)
This system is similar to that used by Illinois and West Virginia Republicans — but there's one crucial difference. Pennsylvania lists these delegate candidates on the ballot without any disclosure of which presidential candidate (if any) they support.
Accordingly, each delegate will be completely unbound at the Republican convention in July, and free to make up his or her own mind. If Trump has fallen just short of a majority, these unbound delegates could choose to put him over the top — or sink him.
Unsurprisingly, there's been a frenzy of activity aimed at trying to suss out just who these various delegate candidates support. Conservative activist Phil Kerpen has put together an excellent spreadsheet tracking the most recent information on their loyalties, and the Morning Call and PennLive.com have similar lists.
It's unclear how many voters will know or care about this tracking, though in past contests, Trump's campaign — based so much on his name and charisma — has stumbled when it comes to electing delegates.
Furthermore, the #NeverTrump movement has endorsed a list of delegates it thinks will vote to block Trump. So if you're trying to track just how well Trump does tonight, keep an eye of how many of these delegates end up winning.