Update: Donald Trump has won the Republican primary in Indiana, according to calls by multiple media outlets. This makes him overwhelmingly likely to win the GOP nomination. The Democratic contest has not yet been called.
Original story: We're now into the final stretch of presidential primary voting, and today both parties' voters will go to the polls in Indiana. Polls close at 6 pm local time (which is Eastern time for most of the state but Central time for its western edge).
For the Republicans, this is a hugely important contest — Donald Trump has the opportunity to hand Ted Cruz a defeat that could all but wrap up the GOP nomination for the billionaire in the eyes of the political world. Trump has led every recent Indiana poll except for one, and he currently leads Ted Cruz by more than 10 points in the RealClearPolitics average.
Now, Trump won't technically be able to clinch a majority of Republican delegates until June 7. But Indiana has been viewed by many as, effectively, Cruz's last stand — because if Trump wins most or all of Indiana's 57 delegates, it will be relatively easy for him to get to that majority in the remaining states. (If Cruz wins, in contrast, Trump's path to that majority will grow more difficult, but not impossible.)
On the Democratic side, the stakes are less high, since Hillary Clinton already has such an advantage in the delegate math, and because the party's Indiana delegates are allotted proportionally. Still, a win for Bernie Sanders could give him a success to point to as he tries to keep his campaign going until California votes on June 7. (Clinton has, on average, a single-digit lead in Indiana polls.)
What to expect on the Republican side
Indiana is so important for the Republicans because it's one of just a few remaining states that at one point looked like they could send either a big delegate haul or nothing at all to Trump — thus either greatly helping or somewhat complicating his quest to win a delegate majority and avoid a contested convention.
After all, Cruz has recently won a solid victory in Wisconsin, and Trump has performed unevenly in the Midwest. Cruz had at least been kinda close in polls. And Indiana allots its delegates winner-take-all (30 to the statewide winner and 27 to winners in congressional districts), so even a narrow defeat for Trump could mean missing out on most of its 57 delegates.
But if those delegates instead go into Trump's column, then considering how the rest of the calendar breaks down he'll have a relatively easy path toward his majority rather than a difficult one. He looks very strong in West Virginia and New Jersey, and is assured of some delegates from the proportional states of Oregon, Washington, and New Mexico.
If all that happens, Trump will only need to win a relatively small number of congressional districts in California to put him over the line — and if he wins big in the Golden State, he'll clinch easily.
Furthermore, if Cruz can't win Indiana considering all he seems to have going for him, there's very little reason to believe he can win California, which is the most important state remaining and where Cruz is even further behind in recent polls.
Cruz may well try to prolong the campaign for another month even if he loses in Indiana, in hopes of a miracle in California. But with Republican elites seemingly more and more resigned that Trump will be their nominee, and lacking the appetite for a protracted convention battle, he'd likely be simply delaying the inevitable.
What to expect on the Democratic side
The Indiana contest isn't as dramatic on the Democratic side for a couple of reasons. First, delegates are allotted proportionally rather than winner-take-all. So if the outcome is close, as the polls currently show, it will make little difference in the delegate tally overall.
Second, Hillary Clinton is currently in a commanding position in the delegate chase. She leads Bernie Sanders by nearly 300 pledged delegates at this point. And to finish with a majority of pledged delegates, she only needs to win 34 percent of those remaining.
In contrast, for Sanders to win a pledged delegate majority, he needs to win 66 percent of the remaining pledged delegates — which seems incredibly difficult given Democrats' proportional rules. So unless Sanders wins more than 66 percent of Indiana's delegates tonight, he'll fall even further behind his target.