Bernie Sanders, who calls himself a democratic socialist, has energized young voters with calls for greater economic equality and denunciations of what he sees as a corrupt US political system.
Hillary Clinton's path to the Democratic nomination has been anything but smooth, but after Bernie Sanders pulled off a political stunner by defeating her in Michigan, she aims to right the ship heading into their Wednesday debate.
Clinton passed the half-way point in the race to the 2,383 delegates needed to win the party's presidential nomination on Tuesday, after she handily defeated Sanders in the southern Gulf state of Mississippi.
But the Vermont senator's upset win in Michigan gave his campaign a needed jolt of momentum, and raised questions about the former secretary of state's ability to win over key industrial states in the general election.
Sanders exulted in a "fantastic night in Michigan," where his broad ground game and higher ad spending helped him capture 49.9 percent of the vote compared to Clinton's 48.2 per cent -- confounding poll predictions.
On Wednesday the two square off for the eighth time, in Miami at 0200 GMT for a debate broadcast on CNN in English and Univision in Spanish. It comes six days before hugely consequential votes in the heavily populated Florida, Ohio and Illinois.
The showdown will give them ample opportunity to discuss the economy, trade and immigration.
The Republicans follow with their own debate Thursday, also in Miami.
Sanders, who calls himself a democratic socialist, has energized young voters with calls for greater economic equality and denunciations of what he sees as a corrupt US political system.
His surprise win in rust belt Michigan, home of America's auto industry, suggests blue collar voters might be hesitant to align with Clinton next Tuesday -- or later in the general election.
But Clinton has won 13 out of 22 nomination contests and despite the Michigan setback, her team remained confident, explaining that her blowout win in Mississippi meant she walked away with the majority of Democratic delegates, and is inching closer to an "insurmountable" delegate lead.
"Wins like the ones that Senator Sanders earned last night in Michigan will simply not be enough for him to truly compete for the nomination," Clinton campaign manager Robby Mook told reporters on a conference call.
Delegate math dictates that Sanders must be competitive in a growing number of states if he wants to best Clinton, Mook added.
"He needs to not just win those states, but he needs to do so by very lopsided margins if he is going to catch up."
Team Sanders countered that if he won in Michigan, he can repeat the performance in the other midwestern industrial states that vote Tuesday -- Ohio, Illinois and to a degree Missouri.
"She's having a difficult time competing outside the South, and there are almost no more southern states so the second half is much more favorable for Bernie Sanders," his campaign manager Jeff Weaver told CNN.
Trump the uniter?Republican frontrunner Donald Trump, meanwhile, emerged strengthened by victories Tuesday in Michigan, Mississippi and Hawaii.
He has now won 15 of 24 races, and looks to next week's primaries that could be crucial to his effort to seize his party's nomination since on the Republican side Florida, Ohio and Illinois are winner-take-all in the delegate race.
It was not a clean Tuesday sweep for Trump, however.
Arch-conservative Texas Senator Ted Cruz won Idaho, bolstering his argument that he is the main alternative to Trump.
On Wednesday Cruz picked up the endorsement of Carly Fiorina, the former Hewlett-Packard executive and ex-Republican presidential hopeful.
Tuesday's losers were Florida Senator Marco Rubio, seen as the best mainstream hope of derailing Trump, and Ohio Governor John Kasich. Both are falling far behind in the delegate count.
The pair face must-wins in their respective home states on March 15. Trump leads polls in both those battlegrounds, and he made clear he aims to snag them both.
"If I win those two I think it's over," Trump told CNN.
The New York real estate mogul's caustic style and incendiary rhetoric has angered some voters and influential Republicans, but he insists he can draw millions more to the party.
There were already signs that Trump -- boosted by his latest primary wins -- was trying to smooth over the wounds opened during the campaign by reaching out to party leaders.
He told CNN he had a "great talk" with Republican House Speaker Paul Ryan, who had previously criticized him.
"He called the other day. He was so great," Trump said. "We should be talking and unifying." Trump described the call as "very conciliatory."
Perhaps seeking to appear more presidential, Trump suggested Thursday's Republican showdown "will be a softer debate," and suggested Republicans should swallow their resentment and rally behind him.
"We have something going that's so good we should grab each other and unify the party, and nobody is going to beat us," he said.