Donald Trump has won two more states, Michigan and Mississippi, in his bid for the White House.
Democratic front-runner Hillary Clinton won her party's primary in Mississippi, but her battle with Bernie Sanders in Michigan is too close to call.
The Republicans are also voting in Idaho and Hawaii.
The four state contests on Tuesday help choose the candidates from each party who will compete in November's presidential election.
"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," Trump said at a press conference in Florida.
He also said he would be more presidential than anybody except Abraham Lincoln and that "no one is more conservative than me".
Hillary Clinton, addressing voters in Ohio, talked about holding businesses accountable for exploiting workers and hoarding profits, and said the election should not be about petty insults.
"Running for president shouldn't be about delivering insults," says Clinton. "It should be about delivering results."
After the previous round of voting on Saturday, Trump, a New York property tycoon with no experience of elected office, won in Louisiana and Kentucky.
Ted Cruz was victorious in Kansas and Maine, while his Republican rival Marco Rubio won in Puerto Rico.
Analysts say Cruz appears to be the only candidate who can stop Trump, who has been fiercely criticised by the Republican establishment.
All eyes are now on Michigan, where Trump is fighting off an unexpectedly strong challenge from Ohio Governor John Kasich.
On the Democratic side, Clinton lost to Bernie Sanders in Kansas and Nebraska at the weekend, but won in Louisiana.
But the early results in from Michigan suggests Sanders is pushing her hard in the state.