Donald Trump could take a giant step toward securing the Republican presidential nomination if he wins the Florida and Ohio primaries, which would intensify pressure for rivals from the party establishment to step down from the race.
Mr Trump has the potential to sweep five big states holding primaries on Tuesday for the November 8 election -- Florida, Ohio, Illinois, North Carolina and Missouri.
He could knock out his two mainstream rivals, Ohio Governor John Kasich and US Senator Marco Rubio of Florida, if he wins their states.
The New York real estate tycoon has a significant lead over Rubio in opinion polls in Florida, but is neck-and-neck with Kasich in Ohio.
Any win by either Rubio, Kasich or US Senator Ted Cruz of Texas would give at least a small degree of hope to Republicans battling to deny Trump the nomination.
Mr Trump said on Tuesday that his momentum was already drawing in establishment Republicans who had previously baulked at his candidacy but now saw him as the likely nominee.
"They're already calling," he told NBC, without naming names. "The biggest people in the party are calling."
Mr Trump victories in the five states could make what once seemed inconceivable a strong probability, putting the 69-year-old who has vowed to deport 11 million illegal immigrants and impose some protectionist trade policies on a glide path to representing the Republican Party in the November election.
Mr Trump drew first blood on Tuesday, winning the Northern Mariana Islands caucuses with almost 73 percent of the vote, local Republican Party executive director Jason Osborne said.
The win in the US Pacific commonwealth gives him nine delegates.
On the Democratic side, former Secretary of State Hillary Clinton can put some distance between herself and rival Bernie Sanders, a US senator from Vermont, in Tuesday's voting.
Opinion polls gave her a big lead in Florida and North Carolina, but showed Mr Sanders gaining ground in Ohio, Illinois and Missouri, a possibly worrisome sign for Ms Clinton after Mr Sanders' surprise victory in Michigan a week ago.