Donald Trump has won two more states, Michigan and Mississippi, in his bid to be the Republican White House pick.
Donald Trump Photo: AFP
In the Democratic race, Bernie Sanders had a surprise victory in Michigan, but Hillary Clinton increased her overall lead with a big Mississippi win.
Ted Cruz won a Republican-only race in Idaho, and Hawaii results come later.
The four states are the latest to choose their candidates from each party who will compete in November's presidential election.
It was a terrible night for Republican Florida Senator Marco Rubio, who came in a distant fourth in both Michigan and Mississippi, a week before his must-win contest in his home state.
Ted Cruz has talked about "a real shift in momentum". Photo: AFP
Mr Trump, a businessman with no experience of elected office, leads the polls in Florida, from where he delivered his victory speech on Tuesday night.
"One of the things I am most happy about is the turnout has been just massive… I think it's the single biggest story in politics today," he said at a press conference in Jupiter.
Donald Trump Photo: AFP
He also said he would be more presidential than anybody except Abraham Lincoln and that "no one is more conservative than me".
The Democratic opponent Mr Trump is most likely to face if he gets the Republican nomination, former US Secretary of State Hillary Clinton, addressed voters in Ohio after her Mississippi win.
"Running for president shouldn't be about delivering insults," said Mrs Clinton, in a thinly veiled dig at the outspoken Mr Trump.
"It should be about delivering results."
Republican presidential candidate Marco Rubio greets people at Tampa Convention Center on March 7. Photo: AFP
Mr Sanders' win in Michigan came as a shock after weeks of polling that suggested Mrs Clinton was well ahead.
"I am grateful to the people of Michigan for defying the pundits and pollsters and giving us their support," Mr Sanders said in a statement following his win.
"This is a critically important night. We came from 30 points down in Michigan and we're seeing the same kind of come-from-behind momentum all across America."
Ohio Governor John Kasich Photo: AFP
Analysts say conservative firebrand Mr Cruz appears to be the only candidate capable of stopping Mr Trump, who has been fiercely criticised by the Republican establishment.
The party's 2012 nominee, Mitt Romney, described Mr Trump as a bully and a fraud who would lose a general election because of his extreme positions on immigration and Islamic State.
A central plank of Mr Trump's campaign is to deport 11m undocumented migrants and build a wall on the southern border, paid for by Mexico.
Hillary Clinton and Bernie Sanders speak at the Democratic presidential candidate in Michigan. Photo: AFP
-CNN / BBC