Democrat Hillary Clinton has won seven states and Donald Trump at least seven in Super Tuesday elections, the biggest day of the primary campaign.
While there were bright spots for other candidates, the front-runners appeared ever more likely to end up in a
general election showdown.
Despite a healthy showing for her rival Bernie Sanders, Clinton began to turn some of her attention to Trump.
"It's clear tonight that the stakes in this election have never been higher and the rhetoric we're hearing on the other side has never been lower," she said in front of supporters in Miami.
Trump responded in kind in Palm Beach, Fla.
"I am a unifier," Trump told reporters, dismissing concerns that his nomination would rip apart the party. "Once we get all this finished, I'm going after one person - Hillary Clinton."
He cast her as part of a political establishment that has failed Americans.
"She's been there for so long," Trump said. "If she hasn't straightened it out by now, she's not going to straighten it out in the next four years."
Trump won a close race in Vermont over John Kasich, but was denied a potential eighth win when Ted Cruz was projected as the winner of a close content in the Republican Alaska caucuses.
Trump also won in Virginia, Alabama, Massachusetts, Tennessee, Arkansas and Georgia.
Massachusetts was the last on the Democratic side to be projected for Clinton. In doing so, she captured all four of the most delegate-rich races on Tuesday, following victories in Georgia, Virginia, and above all, Texas, which comprises 252 delegates.
Clinton, the former senator and secretary of state and First Lady, also won in Arkansas — where her husband Bill was governor before going on to the U.S. presidency — as well as Alabama and Tennessee.
"America never stopped being great. We have to make America whole. We have to fill in what's been hollowed out," Clinton said.
Cruz, who captured the first race in Iowa before Trump embarked on a three-state winning streak, was back on top in his home state of Texas and neighbouring Oklahoma.
Trump mocked Marco Rubio for "having a tough night" because he did not win any states, although the Florida senator would later win in Minnesota.
Trump congratulated Cruz on his wins on Super Tuesday, saying "I know how hard Ted works."
Sanders won his home state of Vermont, as well as in Colorado, Minnesota and Oklahoma.
"This campaign is not just about electing a president. It is about transforming America," Sanders told a crowd at his headquarters in Essex Junction, Vt.
Clinton was assured of winning at least 457 of the 865 delegates at stake on Super Tuesday, while Sanders picked up at least 286 delegates. Overall, Clinton now has at least 1,005 delegates. Sanders had at least 373. It takes 2,383 delegates to win the nomination.
'Prayerfully consider' dropping out: Cruz
Trump was introduced by erstwhile presidential candidate Chris Christie, who dropped out of the race and then endorsed Trump just hours after arguably the businessman's least impressive debate performance last week in Houston.
"Our fight this fall as a united Republican party to make sure Hillary Clinton never gets back into the White House," said Christie, who was then forced to stand aside for a lengthy speech, which transitioned into Trump taking several questions from reporters.
Rubio, along with Cruz, scrambled to block Trump's path to the nomination. Both senators have launched furious verbal attacks on the billionaire businessman in recent days, but some in the party establishment fear the anti-Trump campaign has come too late.
Trump shrugged off Rubio, who has grown strident in recent days, even commenting on the front-runner's "orange" skin.
"I've always liked Marco until about a week ago until he went hostile," said Trump. "Until he became Don Rickles."
Cruz said to his supporters that bringing Trump closer to the nomination would be a "disaster" for the Republican Party.
He highlighted their differences on Israel, the Iranian nuclear deal and the possibility of compromising over a choice for the Supreme Court.
"After tonight we have seen that our campaign is the only campaign that has beaten, that can beat and that will beat Donald Trump," he said.
While Cruz's prediction was proven untrue by a Rubio win in Minnesota about an hour later, he could point to amassing dozens of more delegates than his fellow senator on the night as well as carrying four states so far in the race.
Cruz didn't mention anyone by name but asked fellow candidates to "prayerfully consider" the future of their campaigns in order to mount an unified challenge to Trump.
It was the first state win so far for Rubio. He sought to stay competitive in the delegate count, while hoping to pull off a win in his home state of Florida on March 15, where the winner takes all delegates.
Ohio Gov. John Kasich threatened to win in only one state, running close with Trump in Vermont, but will try and capture his home state, another winner-take-all primary also being held on March 15.
Dr. Ben Carson was an also-ran in each state on Tuesday, but will also try and find favour in Florida.
Carson took the stage Tuesday in Baltimore and called the political system "rotten to the core," wondering where civility had gone.
He told his supporters, "our nation is in horrible trouble. Why sit there and talk about each other and tear each other down when we have such important issues to deal with?"
Republican Party commitment to Trump in question
Trump won at least 203 delegates Tuesday. Cruz collected at least 144 delegates and Rubio picked up at least 71. Overall, Trump leads with 285 delegates, Cruz has 161, Rubio has 87, Kasich has 25 and Carson has eight. It takes 1,237 delegates to win the GOP nomination.
Republicans spent months largely letting Trump go unchallenged, wrongly assuming that his populist appeal with voters would fizzle. Now Republican leaders are divided between those who pledge to fall in line behind him if he wins their party's nomination and others who insist they can never back him.
An Associated Press survey of Republican senators and governors across the country showed just under half of respondents would not commit to backing Trump if he's the nominee. Their reluctance could foreshadow an extraordinary split in the party this fall.
The worries among Republicans appeared to grow after Trump briefly refused to disavow former Ku Klux Klan leader David Duke during a television interview. Trump later said he had not understood the interviewer who first raised the question about Duke, and he did repudiate him.
House Speaker Paul Ryan said Tuesday that anyone who wants to be the Republican presidential nominee must reject any racist group or individual.
"When I see something that runs counter to who we are as a party and a country I will speak up. So today I want to be very clear about something: If a person wants to be the nominee of the Republican Party, there can be no evasion and no games. They must reject any group or cause that is built on bigotry," Ryan said.
Primaries are more like traditional elections, where delegates are selected by secret ballot. Delegates are selected in caucuses by show of hand or separating into groups based on who voters support
States holding voting contests in both parties:
- Alabama (Projected winners: Trump, Clinton)
- Arkansas (Projected winner: Trump, Clinton)
- Georgia (Projected winners: Trump, Clinton)
- Massachusetts (Projected winner: Trump)
- Minnesota (Projected winners: Rubio, Sanders)
- Oklahoma (Projected winner: Cruz, Sanders)
- Tennessee (Projected winners: Trump, Clinton)
- Texas (Projected winners: Cruz, Clinton)
- Vermont (Projected winners: Trump, Sanders)
- Virginia (Projected winners: Trump, Clinton)
Republicans voted in Alaska caucuses and Democrats in Colorado (Sanders).
With files from CBC News