Front-runners Donald Trump and Hillary Clinton are aiming for a sweep of all five Northeastern states holding presidential primaries Tuesday, including the big prize of Pennsylvania.
Connecticut, Delaware, Maryland and Rhode Island also hold primaries on Tuesday. Candidates and outside groups have spent $13.9 million dollars on advertisements in the states, with Clinton and Democratic rival Sen. Bernie Sanders dominating the spending.
Tuesday's contests will be the first since Texas Sen. Ted Cruz and Ohio Gov. John Kasich announced that their campaigns were joining forces in an effort to deny Trump the 1,237 delegates he needs to be the Republican nominee.
Under their new arrangement, Kasich won't compete for votes in Indiana, allowing Cruz to take Trump on head to head in the state's May 3 primary. Cruz will do the same for Kasich in Oregon and New Mexico.
Trump panned his rivals' strategy as "pathetic."
"If you collude in business, or if you collude in the stock market, they put you in jail," Trump said as he campaigned in Rhode Island. "But in politics, because it's a rigged system, because it's a corrupt enterprise, in politics you're allowed to collude."
Cruz and Kasich's public admission of direct coordination was highly unusual and underscored the limited options they now have for stopping the real estate mogul. The effectiveness of the strategy was quickly called into question after Kasich said publicly that while he won't spend resources in Indiana, his supporters in the state should still vote for him.
On the Democratic side, wins for Clinton in most of Tuesday's contests would leave little doubt that she'll be her party's nominee. Sanders' team has sent mixed signals about his standing in the race, with one top adviser suggesting a tough night would push the Vermont senator to reassess his bid and another vowing to fight "all the way to the convention."
Clinton has already been looking past Sanders, barely mentioning him during recent campaign events. Instead, she deepened her attacks on Trump, casting the billionaire businessman as out of touch with Americans.
"If you want to be president of the United States, you've got to get familiar with the United States," Clinton said. "Don't just fly that big jet in and land it and go make a big speech and insult everybody you can think of."
Asked Monday whether she needed to do more to gain Sanders' support in the general election, she noted her loss in the 2008 Democratic primaries to Barack Obama.
"I did not put down conditions," she said on MSNBC. "I said I am supporting Senator Obama. ... I hope that we will see the same this year."
Clinton is on solid footing in the Democratic race and enters Tuesday's contests having accumulated 82 percent of the delegates needed to win her party's nomination. While she can't win enough delegates to officially knock Sanders out of the race this week, she can erase any lingering doubts about her standing.
Trump's path to the nomination remains narrow, requiring him to win 58 percent of the remaining delegates to reach the magic number by the end of the primaries. He's hoping for a solid victory in Pennsylvania, though the state's unique ballot could make it hard for any candidate to win a big majority.
While the statewide Republican winner gets 17 delegates, the other 54 are directly elected by voters and can support any candidate at a convention. Their names are listed on the ballot with no information about which White House hopeful they support.
In addition to Pennsylvania, Democrats are competing for 384 delegates in Tuesday's contests, while Republicans have 172 up for grabs.
The Associated Press contributed to this report.