For Democrat Hillary Clinton and Republican Donald Trump, victories in New York could help quiet critics who have questioned their strength as front-runners.
Each has suffered losses in recent contests that emboldened their rivals, though they still lead in delegate counts and are favoured in Tuesday's New York primary.
Clinton, who represented the state as a senator for eight years, spent the final hours of campaigning trying to drive up turnout among women and minorities, her most ardent supporters.
"We're not taking anything for granted," she said on Monday after greeting workers at the Hi-Tek Car Wash & Lube in Queens.
Clinton has accumulated 1,758 delegates to Vermont Senator Bernie Sanders' 1,076.
Those totals include both pledged delegates from primaries and caucuses, as well as superdelegates, the party insiders who can back the candidate of their choice regardless of how their state votes.
Heading into the New York primary, Sanders needs to win 68 per cent of the remaining delegates if he hopes to clinch the Democratic nomination. It takes 2,383 to win.
On the Republican side, Trump leads with 744 delegates, ahead of Cruz with 545 and Kasich with 144. It takes 1,237 to win the Republican nomination.
Clinton's campaign sees New York as a make-or-break moment for the Democratic race.
A loss in her adopted home state would be a devastating political blow, but a big win would bolster her delegate lead over Sanders and put her closer to becoming the first woman nominated for president by a major political party.
Sanders has rattled off a string of wins in recent primaries and caucuses, but unless he can topple Clinton in a state like New York, where 247 Democratic delegates are up for grabs, he faces increasingly limited opportunities to change the trajectory of the race.
For Trump, New York is an opportunity to rebound from a trying stretch for his campaign.
The biggest question for him heading into Tuesday is whether he captures more than 50 per cent of the vote statewide, which would put him in strong position to win all of the state's 95 Republican delegates.
Trump has spent the past week emphasising his ties to New York, particularly New York City, where he was born and where buildings bear his name.
A big win for Trump is crucial if he hopes to clinch the nomination before the party's convention in July.
If the race isn't settled by then, he faces the very real prospect of losing to Ted Cruz, whose campaign is mastering the complicated process of lining up individual delegates who could shift their support to the Texas senator after the first round of convention balloting.
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