Voting began on Tuesday in North Carolina and Ohio, two of the five states where Republican and Democratic primary contests are being held to choose a nominee for the November US presidential election.
Polls opened in North Carolina and Ohio at 1030 GMT, while polls are set to open in Florida, Illinois and Missouri at 1100 GMT in an event dubbed "Super Tuesday 2."
Among Republicans, Donald Trump is seeking to confirm his status as a front-runner and possibly push one or two of his rivals out of the race. Among Democrats, Hillary Clinton hopes to fend off a resurgent Bernie Sanders and assert her claim as the party's all-but-certain presidential nominee.
Republicans gearing up for a decisive vote in five US states, spotlight would be on the controversial frontrunner Donald Trump as the outcome could potentially seal the party's presidential nomination after one of the most polarised polls in American history.
A win for Trump, 69, as latest polls indicate, could very well bring the billionaire real estate magnate very closer to the magical figure of 1,237 delegates needed the win the party's presidential nomination for the November 8 election.
Given his election rhetoric which has rattled the Republican establishment and the opposition Democratic presidential aspirants, Trump could make it one of the most polarised elections in American history.
Trump, who has so far won primaries and caucuses in 14 States is said to be the favorite in Florida, North Carolina, Illinois and Missouri, while in Ohio he is engaged in a neck and neck fight with the State Governor John Kasich, 63.
Days ahead of the Super Tuesday 2.0, Trump's rallies have been marred by protests and even violence. He had to cancel his election meeting in Chicago on Friday due to clashes between his supporters and protesters. On Saturday, one of the protesters jumped over the fence and reached the stage when Trump was speaking in Dayton Ohio.
Many experts have said this is because of Trump's campaign narrative against a section of the society. His rivals have accused him of inciting violence or indulging in political arson. The White House has alleged that Trump is inflaming tension. Trump, however, strongly refutes these allegations.
His rallies are drawing large crowds as compared to other candidates, despite the fact that top Republican leadership has ganged up against him and are pulling out all their energy to prevent him reaching the magical figure of 1237 delegates.
Under the strategy developed by Mitt Romney, the 2012 presidential candidate of the party, the party establishment is rallying behind Kasich in Ohio and Senator Marco Rubio in Florida, who have the best chances of winning these winner- take-all States.
In Florida 99 delegates are at stake while in Ohio 66 delegates. Illinois has 69 and Missouri has 52 delegates.
In all more than 360 delegates are at stake on what is now being called as Super Tuesday 2.0.
As per this strategy, in Ohio on Monday, Romney campaigned for Kasich, who so far has not registered even a single win. But he hopes to win his home State.
Given the high stakes, Trump last evening addressed a massive rally of thousands of people. Any major loss for Trump today would still keep him ahead of others in terms of delegate count, but would make it difficult for him to get more than half of the 2472 delegates.
In that scenario, which is being pushed by Romney-led establishment, the Republican presidential nominee could be decided at the party's convention in Cleveland in July, which could result in Trump losing out in case of all other candidates and party leaders ganging up against him.
But an impressive Trump win today would negate any chances and the New Yorker could all but become the party's presumptive nominee, a scenario which is scary to many in not only in his party, but also Democrats and the US President Barack Obama. The White House yesterday warned that this could have global implications.
Trump's opponents refer to his remarks of 27 percent Muslims hating the US, building a wall along the Mexico border for which the money would be paid by the Mexico Government, asking key allies like Japan, Germany and South Korea to pay protection money and near daily anti-China rhetoric as dangerous for American foreign policy and its national interest.
However, Trump believes the opposite. He has said such policies would revive American strength and prestige, make the country stronger, bring back jobs and make the economy strong.
His characteristic campaigning style and speeches have been drawing huge crowd across the country, but over the past few days he has increasingly become isolated not only in his party, but also among the country's top leadership. Both sides are now eying on the voters of Florida, Ohio, North Carolina, Illinois and Missouri to take further their agenda.
In a sign of the stakes, Trump canceled a rally Monday in Florida, where he has a 20-point lead over Marco Rubio, the US senator from the Sunshine State, to make a final pitch in Ohio.
Campaigning with Kasich on Monday was Mitt Romney, the 2012 Republican nominee, who has tried to mobilize his party against Trump.
Also campaigning in Ohio was Sanders, who drew about 2,000 people at a rally in Youngstown where he slammed "disastrous" US trade policies over the past 20 years that he said had cost America "millions of decent paying jobs."
"If we have a good vote, and people come out, we're going to win in Ohio," said Sanders, 74.
The clashes that erupted at Trump's Friday rally in Chicago marked an escalation in tensions that have trailed the controversial candidate.
Protesters turned up at his rally in Hickory, North Carolina Monday, but this time, he patiently waited for their chants to subside.
"There's no violence," he said. "It's a movement, it's a love fest."
He acknowledged there was "anger from all sides including from our side" but added: "We're not angry people. We're all good people. There's anger at the incompetence."
Nevertheless, Trump's invective against immigrants, Muslims and Hispanics have become a staple of his campaign, drawing roars of approval from supporters but also increasingly aggressive protests.
On Saturday, a protester rushed the stage as Trump was winding up a speech near Dayton, Ohio.
"I was thinking that Donald Trump is a bully, and he is nothing more than that," Thomas DiMassimo, a 22-year-old college student who was charged with disorderly conduct and inciting panic, told CNN.
Trump has rejected suggestions that his words have created a climate of violence, instead blaming Sanders supporters for sowing trouble — and threatening to respond by sending his own supporters to picket Sanders rallies.
The Vermont senator responded bluntly at a CNN Democratic town hall event: "Donald Trump is a pathological liar."
Trump's Republican rivals also seemed shaken by the ugly turn in a campaign that has for months endured mudslinging and name-calling.
Rubio, who like Kasich faces a do-or-die test Tuesday in his home state, called Trump's language "dangerous."
"If we reach a point in this country where we can't have a debate about politics without it getting to levels of violence and anger," Rubio told CNN, "we're going to lose our republic."
Trump took some final potshots on Twitter late Monday at Kasich — deriding him as "a disaster" for Ohio for favoring trade agreements — and Rubio, claiming he is "weak on illegal immigration" and has "the worst voting record" in the Senate.
"Vote Trump and end this madness!" he wrote.
With inputs from agencies