Dubbed "Super Tuesday 2" by US media, the latest key date of the November 2016 election cycle will see Democratic and Republican contests in the states of Florida, Ohio, Illinois, Missouri and North Carolina.
The first three states are winner-take-all in terms of delegates for the Republican race, the first such contests of the primary season and a major prize as the frontrunner Trump seeks to propel himself towards the 1,237-delegate majority needed to secure the party nomination.
Just as the billionaire was striving to adopt a more presidential tone, the tensions surrounding his high-octane rallies — the target of systematic protests by activists angered by his anti-immigrant rhetoric — have exploded into violence, and thrown the race into turmoil.
Trump forged ahead on the campaign trail through the weekend after calling off a rally in Chicago amid scenes of chaos on Friday. The following day in Ohio brought a fresh security scare as a protester tried to rush on stage towards the candidate.
Sunday, the real estate mogul was back in Ohio, Illinois and Florida, rallying huge crowds of adoring supporters even as he stood accused of unleashing a dangerous through his harsh tone and inflammatory anti-immigrant rhetoric.
"Super Tuesday" will have a limited impact on the Democratic race, where Hillary Clinton has streaked ahead in the delegate account and is already firm favorite to secure the 2,383 needed for the party nomination.
But on the Republican side the stakes could not be higher, with the day expected to further winnow the field down from the current four contenders: Trump, Texas Senator Ted Cruz, Florida Senator Marco Rubio and Ohio Governor John Kasich.
With Kasich and Rubio both facing make-or-break tests in their home states, both have embarked on a bout of frenzied campaigning.
Trump holds a two-digit lead over Rubio in Florida, but Kasich currently beats the billionaire by a whisker in Ohio according to the RealClearPolitics average of polls. A defeat on home turf is likely to be a kiss of death for Kasich, who holds last place in the Republican primaries.
Both Kasich and Rubio have pledged to stand behind their party's primary winner, but the Trump's refusal to tone down his rhetoric and ease tensions has made each candidate more determined than ever to remain in the game.
A visibly unsettled Rubio — who was seen as the Republican establishment's best chance to beat Trump but has disappointed in the race so far — has said that keeping his word about supporting Trump if he wins "is getting harder every day." Cruz, an ultra-conservative with links to the Tea Party movement and a champion of the religious right, is polling in second place nationally and believes a victory over Trump would be within reach if the race were whittled down to two. Trump and Cruz both cast themselves as political outsiders, frequently lambasting the Washington "elites" but Cruz positions himself as an orthodox conservative, while Trump's pitch has been a populist one, at times conservative and at others liberal.
Even if Trump were to prevail in each state Tuesday, he would not reach the threshold of delegates needed to take his party, but it could propel him to an almost insurmountable lead. To date, Trump has 462 delegates, leading Cruz who has 371, Rubio with 165 and Kasich with 63, according to CNN.
- Democratic marathon -
On the Democratic side the former secretary of state Clinton is ahead with 1,244 delegates to Vermont Senator Sanders' 574, according to CNN.
Clinton also has the backing of nearly 500 "superdelegates," top party officials who have a vote at the party convention to pick the nominee.
Both camps insist that no Democratic winner will emerge for several months, until June races in states such as California. Clinton's victories have been concentrated in the American South, where the substantial African-American population has shown the former first lady unwavering support.
Elsewhere, in the Midwest states and industrial regions surrounding the Great Lakes, Sanders, who calls himself a democratic socialist, has snatched up victories with promises of a political revolution.
Sanders has energized young voters with calls for greater economic equality and denunciations of what he sees as a corrupt US political system.
His protectionist rhetoric has hit home particularly in Michigan, the heart of the automotive industry where he clinched an upset win over Clinton.
On Saturday, Clinton sharpened her opposition to the Trans-Pacific Partnership signed by President Barack Obama as she spoke in Ohio. Sanders is vehemently opposed to the trade deal.