Voting began yesterday across Florida, Ohio and three other states in what has been billed as the most pivotal day of the United States primary race to date.
A slow but steady stream of voters headed into Miami City Hall throughout the morning, among them Mr Zach Ohanna, 65, who works in real estate.
"I voted for Donald Trump because I think we need someone who can change the system," he said.
Graduate student Greg Koman, 35, said he voted for Democratic Senator Bernie Sanders because that was the candidate with the best plan for climate change. "I think he is the only one really paying attention to the problem and it is something I care about," he said.
Unlike Missouri, Illinois and North Carolina, which also vote on Tuesday, Florida and Ohio are significant because of their winner-take-all status.
Candidates seek to win delegates in each state to capture their party's nomination at the conventions held in July. But in Florida and Ohio, the winning candidates automatically win all of the state's delegates.
So, wins in these two states could herald a turning point in the campaigns of Florida Senator Marco Rubio and Ohio governor John Kasich, while momentarily halting Republican front-runner Donald Trump.
The businessman cancelled a Florida rally to focus on Ohio, believing he had wrapped up the race in the southern state. A poll released on Monday by Quinnipiac University showed Mr Trump ahead of Mr Rubio by 24 percentage points.
Mr Rubio "looks like he'll soon be toast," said assistant director Peter A. Brown of the Quinnipiac poll .
Mr Rubio has held a series of events all through the state, including a homecoming event in West Miami at the basketball court where he used to play as a kid.
While some in the crowd were bullish about his chances, others offered this advice: "If he doesn't win tomorrow, he can stop his campaign and focus on becoming governor. Then he can try again," said Mr Michael Barrios, 38, a nurse.
Over in Ohio, Governor John Kasich campaigned with 2012 Republican presidential nominee Mitt Romney, while Senator Ted Cruz decided to train his sights on Illinois.
Mr Kasich and Mr Trump are neck and neck in Ohio - the only state where Mr Trump is not ahead.
Appealing to Ohio voters, Mr Romney, who has emerged as the de facto leader of the "Never Trump" movement, told the crowd at a town-hall event in North Canton: "This is the guy you need to vote for. America is counting on you... unlike other people running, he has a real track record."
Meanwhile, Mr Trump, who spoke in Youngstown, Ohio, characterised Mr Romney as "a disaster" for not winning the 2012 election and said Mr Kasich was "overrated" as a governor. He made appearances in North Carolina and Florida as well.
Tight races are also expected for the Democrats in three of the five states, the exceptions being Florida and North Carolina, where Mrs Hillary Clinton leads by 28.6 percentage points and 24 percentage points respectively, according to political website RealClearPolitics.
After the shock upset in Michigan last week, where she was supposed to bag an easy win, the Sanders campaign has gained momentum, and hopes for a strong showing on Tuesday.
In Ohio, where Mr Sanders trails by just 5 percentage points, he spoke of a "rigged economy" at his rally, emphasising that "almost all new income and wealth is going to the top 1 per cent".
He also travelled to Illinois, North Carolina and Missouri - where he is supposed to have a slight edge over Mrs Clinton, who made appearances in Illinois and North Carolina.
In Illinois, Mrs Clinton, targeted unions and minority voters, who have helped her win earlier primary states.
Calling on her supporters to get out the vote, she said: "Please do everything you can in the next 24-plus hours so we can come out of these elections with a wind at our backs, so we can start talking about unifying the Democratic Party and unifying our country."
The last poll will close at 8pm (8am today in Singapore).
- Additional reporting by Jeremy Au Yong