Hillary Clinton and Donald Trump have moved further ahead of their rivals in elections that will help to decide the US presidential nominees of the Democratic and Republican parties.
Republican front-runner Mr Trump swept three states and drove rival Marco Rubio out of the White House race, but a loss in the crucial state of Ohio has created a dilemma for the party.
While the Republican race remained in turmoil, Mrs Clinton's victories in Florida, Illinois, Ohio and North Carolina cast doubt on US Senator Bernie Sanders' ability to overtake her for the Democratic Party's nomination.
Mr Trump's wins in Florida, Illinois and North Carolina brought him closer to the 1,237 delegates he needs to win the nomination and left those in the party trying to stop him with a dilemma.
Republicans can either throw their weight behind a candidate who rejects their policy goals or go on trying to stop him in the hope that he falls short of the majority required.
If that happens it will enable them to put forward another candidate at the July convention in Cleveland to formally pick their candidate for the 8 November election.
That, however, would risk alienating the millions of Americans backing the New York billionaire.
Marco Rubio drops out of Republican Presidential race after defeat in home state of Floridahttps://t.co/QlB5Z31eMe— RTÉ News (@rtenews) March 16, 2016
The wins for former Secretary of State Mrs Clinton added to her lead in pledged delegates over Senator Sanders of Vermont, and gave her an almost insurmountable edge.
As she had after other primary wins, Mrs Clinton was thinking about a possible match-up in the presidential election with Mr Trump.
"We can't lose what made America great in the first place, and this isn't just about Donald Trump," Mrs Clinton told supporters in West Palm Beach, Florida.
"We can't just talk about economic inequality, we have to take on all forms of inequality and discrimination."
Ohio Governor John Kasich's victory in his home state left him as the last establishment Republican candidate standing after Mr Rubio, a US senator, pulled out of the race after losing in his home state of Florida.
US Senator Ted Cruz of Texas, who has sought to fashion himself the lead anti-Trump alternative, was outclassed by Mr Trump everywhere except Missouri, where he trailed him narrowly with nearly all votes counted.
MSNBC projected Mr Trump and Mrs Clinton would win Missouri in very tight races.
Mr Trump, speaking at his Mar-a-Lago beach front resort on Florida's Atlantic Ocean coast, called on Republicans to unite behind him.
After yesterday’s victories, Mr Trump needs to win about 54% of the roughly 1,100 delegates still up for grabs.
It is not an insurmountable challenge, especially when factoring in winner-take-all states, like Arizona's 58 delegates and New Jersey's 51 delegates.
Mr Kasich's win in Ohio, his first in the nominating fight, makes him the candidate of choice for party leaders worried Mr Trump's rowdy campaign will lead Republicans to defeat not only in the presidential race, but in state and US congressional races.