Trump and Clinton aim for clean sweeps in north-eastern primaries
Donald Trump is aiming for a clean sweep of all five north-eastern states holding primaries on Tuesday, including Pennsylvania, leaving his rivals pinning their hopes of stopping the Republican front-runner with a fragile co-ordination strategy in the next rounds of voting.
For Democratic leader Hillary Clinton, wins in most of Tuesday's contests would leave little doubt that she will be her party's nominee.
Rival Bernie 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".
Mrs Clinton was already looking past Mr Sanders, barely mentioning him during recent campaign events. Instead, she deepened her attacks on Mr 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," she 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 on Monday whether she needed to do more to gain Mr 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."
In addition to Pennsylvania, Connecticut, Delaware, Maryland and Rhode Island hold primaries on Tuesday. Candidates and outside groups have spent 13.9 million dollars (£9.6 million) on advertisements in the states, with Mrs Clinton and Mr Sanders dominating the spending.
Democrats are competing for 384 delegates in Tuesday's contests, while Republicans have 172 up for grabs.
The Democratic race is far more settled than the chaotic GOP contest, despite Mr Trump having a lead in the delegate count. The businessman is the only one left in the race who can reach the 1,237 delegates needed to clinch the nomination before the convention, but he could very well fall short, pushing the nominating process to the party's July gathering in Cleveland.
Texas Senator Ted Cruz and Ohio Governor John Kasich are now joining forces to try to make that happen. Their loose alliance marks a stunning shift in particular for Mr Cruz, who has called on Mr Kasich to drop out of the race and has confidently touted the strength of his convention strategy.
Mr Kasich has won just a single primary - his home state - but hopes to convince convention delegates that he is the only Republican capable of defeating Mrs Clinton in the general election.
Under the new arrangement, Mr Kasich will not compete for votes in Indiana, allowing Mr Cruz to take Mr Trump on head to head in the state's May 3 primary. Mr Cruz will do the same for Mr Kasich in Oregon and New Mexico.
"It is big news today that John Kasich has decided to pull out of Indiana to give us a head-to-head contest with Donald Trump," Mr Cruz told reporters as he campaigned in Indiana on Monday.
"That is good for the men and women of Indiana. It's good for the country to have a clear and direct choice."
Mr Trump bra\nded his rivals' strategy "pathetic".
"If you collude in business, or if you collude in the stock market, they put you in jail," he 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."
Mr Cruz and Mr Kasich's public admission of direct co-ordination was highly unusual and underlined the limited options they now have for stopping the real estate mogul.
The effectiveness of the strategy was quickly called into question after Mr Kasich said publicly that while he will not spend resources in Indiana, his supporters in the state should still vote for him.
Mr Trump's path to the nomination remains narrow, requiring him to win 58% of the remaining delegates to reach the magic number by the end of the primaries. He is 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.
Mrs Clinton is on solid footing in the Democratic race and enters Tuesday's contests having accumulated 82% of the delegates needed to win her party's nomination.
While she cannot win enough delegates to officially knock Mr Sanders out of the race this week, she can erase any lingering doubts about her standing.