And if they meet for a one-on-one, you can expect fireworks, with Trump having attacked Clinton last night after he won all five primaries.
Speaking at Trump Tower in New York, Trump said: "I think the only card she has is the woman’s card. She’s got nothing else going.
"And frankly, if Hillary Clinton were a man, I don’t think she’d get five per cent of the vote. The only thing she’s got going is the woman’s card, and the beautiful thing is women don’t like her, Okay?"
Trump added: "She will not be a good president. She does not have the strength or the stamina to deal with China or other things ... Hillary will be horrible, absolutely horrible, on economic development. She knows nothing about jobs, apart from jobs for herself."
While Trump labelled himself the "presumptive nominee" with victories in Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island, Clinton was denied a clean sweep by rival Bernie Sanders.
Sanders took Rhode Island, vowing to fight to the end of the nomination process.
Clinton said in her speech: "The other day, Trump accused me of playing the woman card. Well, if fighting women's healthcare and paid family leave and equal pay is playing the woman card, then deal me in."
Trump's victory has led him closer to the nomination after speculation that he might not win an outright majority before the GOP National Convention. If he does not win a majority, the Republican party could theoretically pick another candidate.
It's thought that either John Kasich or Ted Cruz, Trump's rivals, will need to win Indiana - which offers 57 delegates - to stand any chance of stopping Trump from securing that all-important majority.
"Whilst Trump was expected to win all of the states last night, he did even better than expected in terms of vote share and delegates, making it more likely that he'll win outright before a convention," said Matthew Shaddick of Ladbrokes.
Testament to the importance of the victories, the bookie has shortened the odds on Trump to become president, offering 7/2. Meanwhile, Betway have also shortened him from 7/2 to 3/1 to enter the White House next, and shifted him to 1/4 from 1/3 to win the nomination.
Still, Clinton remains the out-and-out favourite to succeed Barack Obama, with many Republicans fearful that Trump would wither against the former secretary of state.