Donald Trump and Hillary Clinton have tightened their grip on the White House race after critical victories in five state primaries.
Florida Senator Marco Rubio dropped out of the Republican contest after a landslide defeat in his home state to front runner Mr Trump.
The 69-year-old billionaire also won Illinois and North Carolina, but lost the Midwestern presidential bellwether of Ohio to John Kasich .
A crestfallen Mr Rubio, once the party favourite, told supporters in Miami: “We will not be on the winning side.“
Mr Trump was neck and neck with his only other rival, Texas SenatorTed Cruz , in a Missouri cliffhanger.
On the Democratic side, Mrs Clinton is claiming a clean sweep in Florida, Ohio, Illinois, North Carolina and Missouri.
Vermont Senator Bernie Sanders battled the former secretary of state to a photo finish in Missouri and her native state of Illinois.
But his hopes of thwarting 68-year-old Mrs Clinton’s march to the Democratic nomination look to have been dealt a serious blow.
“Our commander in chief needs to defend our country, not embarrass it,” she said in her victory speech, an apparent swipe at Mr Trump.
The Republican Party establishment will be relieved their own front runner was beaten by Ohio Governor Mr Kasich in that state.
Conservative grandees fear Mr Trump’s incendiary rhetoric will lead them to oblivion in November’s White House election.
However, analysts say Mr Kasich, with only one victory under his belt, currently has no path to the nomination.
His sole win could even end up aiding Mr Trump by dividing the vote against the brash New Yorker.
The property magnate has taken another stride towards the Republican nomination, a scenario that seemed laughable to pundits only a year ago.
However, he could still fall short of the 1,237 delegates needed to be anointed the party’s White House candidate in July.
The messy Republican fight raises the extremely rare prospect of a contested convention, which could tear apart an already deeply fractured party.
About two-thirds of Republican voters in all five states that held primaries on Tuesday said they back Mr Trump’s plan to stop Muslims entering the US, according to AP news agency exit polls.
Democratic President Barack Obama took a not-too-subtle jab at Mr Trump on Tuesday , saying he was dismayed by the rhetoric on the campaign trail.