Fighting for Florida and beyond, Hillary Clinton and Bernie Sanders tangled repeatedly in their eighth U.S. presidential debate Wednesday night over who's a true advocate for Latinos and who has a track record of letting Hispanics down.

Facing off just six days before Florida gives its verdict on the presidential race, Clinton faulted Sanders for repeatedly voting against a 2007 comprehensive immigration reform bill; he faulted her for opposing a 2007 effort to allow people who were in the country illegally to obtain driver's licences.

Had the immigration package passed back then, Clinton said, "a lot of the issues we are still discussing today would be in the rearview mirror."

Sanders retorted that he opposed the legislation because it included a guest worker program "akin to slavery."

Dem 2016 Debate

Democratic presidential hopefuls Bernie Sanders and Hillary Clinton battled it out on issues including immigration, federal bailouts, jobs, health care and education during Wednesday night's debate in Miami. (Wilfredo Lee/Associated Press)

Clinton has won 762 pledged delegates compared to 549 for Sanders, with 10 delegates from recent primaries still to be allocated. When superdelegates are included, Clinton leads 1,223 to 574, more than halfway to the 2,383 needed to win the Democratic nomination.

On the Republican side Wednesday, billionaire businessman Donald Trump called for Republicans to rally behind his candidacy after he won primaries in three more states, declaring that he could not be defeated in the November general election as the standard-bearer of a united party.

Both Sanders and Clinton were bidding for momentum after Sanders startled Clinton with an upset victory in Michigan on Tuesday. Both candidates laid out rival paths to the Democratic nomination.

Clinton stressed that she has a strong lead in the delegates, declaring, "This is a marathon, and it is a marathon that can only be carried by the kind of campaign I am running."

Sanders said he'd come a long way from the early days when his campaign was largely written off and said his showing in Michigan was evidence that his message is resonating.

Clinton kept pushing on immigration matters, accusing Sanders of supporting legislation that would have led to indefinite detention of people facing deportation, and for standing with Minutemen vigilantes. Sanders called that notion "ridiculous" and "absurd," and accused Clinton of picking small pieces out of big legislative packages to distort his voting record.

The debate opened with a question that appeared to startle Clinton.

Univision's Jorge Ramos asked her if she would drop out of the race if indicted over the handling of her email while secretary of state.

"Oh for goodness [sake], that is not going to happen," Clinton declared. "I'm not even answering that question."

The FBI is investigating the possibility of mishandling of sensitive information that passed through Clinton's private email server.

Sanders, as he has in the past, declined to bite on the issue, saying, "The process will take its course." He said he'd rather talk about the issues of wealth and income inequality.

The candidates squared off days after a testy debate in Michigan on Sunday in which they argued about trade and economic issues of particular interest in the industrial Midwest.

With Missouri, Illinois and Ohio among the states that will be voting on Tuesday, the candidates returned to a pointed matter they'd already argued about three days earlier, scuffling over Sanders' vote against 2009 legislation that bailed out the auto industry, among others. Sanders said it was those "others" — big banks that had fuelled the recession to begin with — that inspired him to vote no. Clinton stressed she'd made a different judgment to side with the automakers.

Immigration commanded considerable attention for good reason: Florida is home to nearly 1.8 million Hispanics, including about 15 per cent of the state's Democrats.

A good share of those Florida voters already have locked in their decisions: nearly 487,000 Democrats have cast early ballots, representing about 11 per cent of registered Democrats.

Hispanic voters have made up about 10 per cent of voters in the Democratic primaries so far this year, and Clinton has been getting about two-thirds of their votes to about one-third for Sanders. The Vermont senator, for his part, stresses that he's making progress on winning over younger Hispanics.

Overall, 691 delegates are at stake on Tuesday, including 99 in Florida, which awards all its delegates to the winner rather than dividing them up proportionately.