The Republican front-runner split four contests on Saturday with rival Ted Cruz, who positioned himself as the prime alternative to Trump in the race for the party's nomination in the Nov 8 election.
Michigan is the biggest prize up for grabs in Tuesday's contests. Most opinion polls show Trump, a billionaire New York real estate magnate and former reality TV star, hanging on to a solid double-digit lead there over Cruz, a US senator from Texas, and Ohio Governor John Kasich, who has climbed in some opinion polls in the Midwestern state neighboring his own.
While Kasich, 63, is in last place in the number of delegates amassed, which are needed to clinch the nomination at the party's July convention, a strong showing for him in the state could further complicate the math for any one anti-Trump candidate.
US Senator Marco Rubio of Florida is the favorite of a Republican establishment alarmed by Trump's controversial proposals and crude style.
Marco Rubio Source: AAP
But Rubio, 44, lags behind and is seen needing a breakthrough win in his home state next week to keep his campaign alive.
Trump, 69, faced a week of blistering attacks from the party's establishment, including from the 2012 Republican nominee Mitt Romney, that ended with a mixed showing in Saturday's contests in Louisiana, Kentucky, Kansas and Maine.
That encouraged some Republican leaders and donors who are trying to block him from a clear shot at the party nomination.
Florida Ads
Anti-Trump Super PACS have spent millions in advertisements and direct mail messaging designed to attack Trump's character in Florida, a state Rubio calls home and Trump calls a second home.
Florida's 99 delegates are awarded on a winner-take-all basis.
"They are trying and they're spending millions of dollars but I have a tremendous following," Trump said Tuesday on Fox News, taking credit for the massive Republican voter turnout in the 2016 campaign.
"That's what's happening - there's life now in the Republican Party," he added.
Many mainstream Republicans have been offended by Trump's statements on Muslims, immigrants and women and alarmed by his threats to international trade deals.
Trump, whose foreign policy proposals have been attacked by some as unworkable, said on Tuesday he did not yet have a foreign policy team. He dismissed criticism that his statements on immigration and trade would damage America's standing on the world stage.
Conservatives meeting in recent days at the Republican Governors Association retreat in Park City, Utah, and a think tank forum in Sea Island, Georgia, said Trump is vulnerable to another blast of attacks before a big day of voting on March 15, the Washington Post reported.
The American Enterprise Institute's World Forum in Georgia over the weekend, which was closed to media, drew Apple CEO Tim Cook, Google co-founder Larry Page, Space-X mogul Elon Musk, as well as congressional leaders, according to the Huffington Post.
"A specter was haunting the World Forum - the specter of Donald Trump," Weekly Standard editor Bill Kristol wrote in an email from the weekend meeting, according to Huffington Post.
"There was much unhappiness about his emergence, a good deal of talk, some of it insightful and thoughtful, about why he's done so well, and many expressions of hope that he would be defeated," wrote Kristol, an influential conservative.
On Tuesday, Republicans and Democrats will also vote in Mississippi, and Republicans in Idaho and Hawaii will make their choices on a day when 150 Republican delegates and 166 Democratic delegates will be up for grabs.
But the focus is the industrial battleground of Michigan, where Trump's relentless anti-free trade rhetoric and promise to slap taxes on cars and parts shipped in from Mexico has resonated in a state that has lost tens of thousands of manufacturing and auto industry jobs.
Among Democrats, Hillary Clinton, 68, also has a solid double-digit lead in Michigan opinion polls over rival Bernie Sanders, 74, a U.S. senator from Vermont. The former secretary of state is also expected to do well in Mississippi, where the Democratic electorate will be dominated by black voters who have overwhelmingly favored her over Sanders.
Big Michigan wins for Trump and Clinton would set them up for a potentially decisive day of voting on March 15 in Ohio, Florida, Illinois, Missouri and North Carolina.
If Trump could sweep the Republican contests in Florida and Ohio and pile up delegates elsewhere, it would probably knock Rubio and Kasich out of the race and make it tough for Cruz, 45, to catch him.
