Five US states are holding primary elections for November’s presidential poll, in a crucial round that could give Democrat and Republican front-runners a chance to cement their leads.
Polls have opened in Ohio and Florida – both deemed key states – as well as in North Carolina, Illinois and Missouri.
Democratic front-runner Hillary Clinton will hope to fend off a challenge from her resurgent rival, Bernie Sanders.
Meanwhile Donald Trump will aim to edge out his Republican rivals in the race.
The New York real estate mogul is the favourite to win his party’s nomination but has run into fierce opposition from within the Republican establishment, as well as facing condemnation from the Democrats.
He scored an early success on Tuesday when he secured all nine delegates in the Northern Mariana Islands, part of the US Pacific Commonwealth, after winning almost 73% of the Republican caucus there.
Victories for any of Mr Trump’s rivals – Ted Cruz, Marco Rubio, or John Kasich – in this round of primaries would give hope to the Republicans fighting to block him.
But polls have given Mr Trump a healthy showing in all five mainland states.
Two primaries seen as most crucial are in the winner-takes-all states of Florida, offering 99 delegates, and Ohio, with its 66 delegates.
Senator Mr Rubio, currently in third place for the Republican Party nomination, has said he must win in Florida, his home state, in order to stop Mr Trump from “hijacking” his party’s nomination, but polls show Mr Trump is slightly ahead.
However, he is trailing Mr Kasich in Ohio, where the fourth-place candidate for the nomination is governor.
For the Democrat nomination, Mrs Clinton begins Tuesday with a comfortable lead ahead of her rival, Vermont Senator Bernie Sanders.
Opinion polls give her a big lead in Florida and North Carolina, but show Mr Sanders is gaining ground in the other three states.
Mr Sanders pulled off a major political upset with his win in Michigan a week ago.
Both Democrat and Republican candidates spoke out against Mr Trump after a string of violence at his rallies.
Texas Senator Cruz, who is in second place for the Republican nomination, blamed Mr Trump for the violence, saying “any candidate is responsible for the culture of the campaign”. Mr Rubio said it was “harder every day to justify” backing Mr Trump.
Mr Trump had blamed Mr Sanders’ supporters for the violence, and said his campaign is “not provoking” and “wants peace”. Mr Sanders accused of him of being a “pathological liar”.