The US Northeastern Primary comes as Ted Cruz and John Kasich attempt to join forces against Republican presidential front-runner Donald Trump and Democrat Hillary Clinton aims to shut the door completely on Bernie Sanders
Here are five things to watch as the results roll in this afternoon.
1 Does the Cruz-Kasich divide continue?
Today's primary contests in Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island are all in states that President Barack Obama won in 2012, and Trump is a favourite to win the GOP race in each (in some, by a large margin). But why?
Trump's vote share has not been much higher in Democrat blue states (36 per cent) than Republican red states (33 per cent), but in several contests his opponents' supporters have been unable to unite around a single candidate, leaving Trump with the lion's share of delegates.
Take Michigan, where Cruz and Kasich each won about 25 per cent of the vote, allowing Trump to win with 37 per cent support even though nearly half of voters said they would be dissatisfied with Trump becoming the nominee, according to network exit polling.
Looking across all contests, Kasich has performed nine percentage points better in Obama's 2012 states than red states, while it's the reverse for Cruz, who has 13 points more support in states Mitt Romney won. But Kasich's better vote share in more Democratic-leaning areas has not been enough to win anywhere beyond his home state of Ohio, and Cruz has lost several close red-state contests in which Kasich's support was weak but larger than Trump's winning margin.
Voters' refusal to unite strategically in opposition to Trump may signal a difficulty for Cruz voters joining Kasich's more moderate ranks (and vice versa), tolerance of Trump becoming the nominee or a doubt that Cruz or Kasich can actually defeat him.
Of course, if Trump wins states with actual majorities - as it looks as though he might on Tuesday - none of this matters. Either way, though, Monday's deal makes it clear that this kind of thing is what Cruz and Kasich would very much like to end.
2 Can Trump basically run the table?
Trump needs to win about 63 per cent of all remaining bound Republican delegates to clinch the party's nomination, and he is poised to far surpass that pace today. The delegate allocation rules differ in each of the five states, but here's a brief state-by-state rundown of what to expect.
- Maryland: Trump has a shot at sweeping all of Maryland's 38 delegates, but is headed for a large windfall no matter what. Trump's double-digit lead in recent polls makes him a strong favorite to secure 14 statewide delegates, which are all awarded to the top vote-getter. The remaining 24 delegates are allocated to the winner in each congressional district, a test of whether Trump's statewide support will hold steady in both urban and rural parts of the state. If Trump wins every district, he wins every delegate.
- Connecticut: Trump has a chance to secure nearly all of the state's 28 delegates. The latest Quinnipiac University poll found him with 48 per cent of likely Republican primary voters to Kasich's 28 per cent and Cruz's 19 per cent. Trump would take all 13 statewide delegates if he can rise above 50 per cent statewide, and he also stands to perform well in congressional districts where the winner is awarded all three delegates, even with less than majority support.
- Pennsylvania: Although Pennsylvania has the largest number of delegates up for grabs today, it's more of a wild card than the other states. Of the Keystone State's 71 delegates, 54 are elected directly by voters and are not bound to support any particular candidate at the convention - although many have said they'll support whoever wins their districts. An additional 17 statewide delegates are bound to whoever receives the most votes in the statewide primary, which looks like it will be Trump. A recent NBC News-Wall Street Journal-Marist poll found Trump leading with 45 per cent to 27 per cent for Cruz and 24 per cent for Kasich.
- Rhode Island: Trump is likely to pick up the largest share of the state's meager number of delegates - 19 - but will not run the table, with delegates awarded proportionally to candidates who garner at least 10 per cent support. A similar process will play out in the state's two congressional districts. A Brown University survey last week found Trump leading with 38 per cent support to Kasich's 25 per cent and Cruz's 14 (a large 17 per cent were undecided in that survey). More recent automated polling suggests a larger Trump edge.
- Delaware: Trump also stands a good chance sweeping Delaware, but it's the smallest prize of the day. The First State has 16 delegates, and if a presidential contender receives more votes than the others, he gets all the delegates. Although there is little reliable polling in Delaware, his advantages in neighbouring Maryland and Pennsylvania are a sign that he should perform well here, too.
3 Who gets second and third?
Trump is the favourite in all five states, but there's fairly close competition for second place on the Republican side that may help Cruz or Kasich make their case as the top Trump alternative.
Polls show Kasich standing in second position in Connecticut, Maryland, Pennsylvania and Rhode Island, but Cruz - who is miles ahead of Kasich in the delegate chase - is within striking distance in each. Cruz's challenge is that today's electorates tend to be light on strongly conservative Republicans and evangelical Christians, two groups where he has performed best.
4 Will black voters doom Bernie Sanders again?
African American voters have averaged one-fourth of the electorate across Democratic primaries, and black Democrats make up a significant share of the electorate in today's contests, which should help Clinton bolster her delegate lead.
In 2008, 37 per cent of Maryland Democratic primary voters were black, as were 28 per cent in Delaware. They made up a smaller share in Pennsylvania (15 per cent), Connecticut (9 per cent) and Rhode Island (7 per cent). Sanders has hoped his message on income inequality would resonate across racial lines, but this year black Democrats have voted for Clinton over Sanders by a 59-percentage-point margin (79-20).
Perhaps more worrying for Sanders is his relatively weak standing in northeastern contests in which he had excelled before. Clinton led Sanders by nine points in a Connecticut Quinnipiac University poll this month and had a similar edge in Rhode Island, according to a Brown University poll (a PPP survey showed Sanders with a narrow edge).
Sanders' lack of any clear edge in states that play to his strengths is a big reason that his hopes of catching up to Clinton in delegates have becoming increasingly impossible.
5 How much will women boost Clinton?
Besides the Democratic contest's sharp generational divide, gender has been a consistent dividing line. Clinton has held a 24-point average lead ahead of Sanders among female voters, while Clinton has won men by a narrower four points in states where exit polls were conducted.
Sanders' challenge is that women have made up a clear 58 per cent majority of voters across 2008 and 2016 primaries. Eight years ago, women made up 62 per cent of primary voters in Maryland, 60 per cent in Delaware, 59 per cent in Connecticut, 58 per cent in Pennsylvania and 57 per cent in Rhode Island.