By Lisa Lerer and Kathleen Ronayne
Tensions have frayed in both the Democratic and Republican presidential races, as Hillary Clinton and Donald Trump try to stave off the prospect of a lengthy battle for the nomination with big victories in New York.
While Clinton escalated her attacks against rival Bernie Sanders, Republican frontrunner Donald Trump complained about a "rigged" nomination process, prompting a fierce defence from party leaders.
Both candidates are pushing for big wins in next week's New York primary, hoping to create a sense of inevitably around their candidacies with sizeable delegate gains.
Campaigning in southern California, Texas Senator Ted Cruz described Trump's attacks on the Republican nomination process as "whining".
"Donald has been yelling and screaming. A lot of whining. I'm sure some cursing. And some late-night fevered tweeting," Cruz told hundreds of supporters gathered in Irvine, California.
He noted Trump's complaints follow his struggles in recent primary contests in Utah, North Dakota, Wisconsin and Colorado.
Trump has seized upon his delegate woes in recent days as evidence that "the fix" was in. He angrily denounced the allocation of all of Colorado's delegates to Cruz as "dirty and disgusting" during a Monday night rally in Albany, New York.
Trump said a similar game was playing out on the Democratic side, where "Bernie Sanders wins and wins and wins" but yet "can't win the race".
"The system is rigged, folks," said Trump, who spoke to more than 10,000 people in an Albany arena during a rally that was interrupted several times by protesters.
Trump's accusations come as he seeks to out-manoeuvre Cruz in local state gatherings where the delegates who will attend the (northern) summer convention are being chosen.
In state after state, Cruz's campaign has implemented a more strategic approach to picking up delegates, which, despite Trump's current lead, are essential if he wants to reach the 1237 delegates needed to win the nomination.
Cruz is pinning his hopes on a contested convention with no candidate having enough delegates to win on the first ballot. On subsequent ballots, many of the pledged delegates will become free to vote for any candidate.
Trump's complaints call into question the integrity of the voting process at a time when the party could be working to unify behind its front runner.
In an interview with conservative radio host Mike Gallagher, Reince Priebus, chairman of the Republican National Committee, pushed back against Trump's claims, saying the convention system used in Colorado is "not an affront to the people of Colorado. It just is what the rule is".
"I don't know why a majority is such a difficult concept for some people to accept," he said.
On the Democratic side, the April 19 primary in New York has become a make-or-break moment for the Clinton and Sanders campaigns.
Clinton hopes to capture what her team says would be an all-but-insurmountable lead by the end of the month in primaries in New York and other eastern states.
Sanders believes he can turn a string of primary wins into a victory in delegate-rich New York. But he faces the daunting challenge of needing to win 68 per cent of the remaining delegates and uncommitted superdelegates, party officials who can vote for any candidate, if he hopes to clinch the Democratic nomination.
That would require blow-out victories in upcoming states, big and small, including New York.
The two Democrats will meet on stage for the first Democratic primary debate in more than a month on Saturday. Since their last face-off, the contest has taken a decidedly negative turn, with the two candidates trading a series of barbs over their qualifications for the White House.