US President Joe Biden and former president Donald Trump have swept to victory in statewide nominating contests across the country, setting up a historic rematch in November's general election despite low approval ratings for both candidates.
Trump won the Republican votes in 14 of 15 states - including delegate-rich California and Texas - brushing aside former UN ambassador Nikki Haley, his last remaining rival.
Haley's only win of the night came in Vermont.
After dominating across 15 states where more than one-third of Republican delegates were up for grabs on Super Tuesday, Trump had all but clinched his third consecutive presidential nomination, despite facing a litany of criminal charges.
Trump and Biden trained their focus on each other as the results became clear.
In a victory speech at his Mar-A-Lago estate in Florida, Trump attacked Biden's immigration policies and called him the "worst president" in history.
"November 5th is going to go down as the single most important day in the history of our country," Trump said.
In a statement, Biden again cast Trump as a threat to American democracy.
"Tonight's results leave the American people with a clear choice: are we going to keep moving forward or will we allow Donald Trump to drag us backwards into the chaos, division, and darkness that defined his term in office?" Biden said.
Biden sailed through the Democratic contests, although a protest vote in Minnesota and six other states organised by activists opposed to his forceful support of Israel in its war against Hamas attracted unexpectedly strong results.
The "uncommitted" vote in Minnesota stood at 19 per cent with nearly 90 per cent of the votes counted, according to Edison Research, higher than the 13 per cent that a similar effort in Michigan drew last week.
Biden nevertheless won Minnesota and 14 other states, including a mail-in vote in Iowa.
He suffered one loss, in the US territory of American Samoa's caucus, where entrepreneur Jason Palmer won 51 votes to Biden's 40.
Another campaign between Trump, 77, and Biden, 81 - the first repeat US presidential match-up since 1956 - is one few Americans seem to want.
Opinion polls show both Biden and Trump have low approval ratings among voters.
Immigration and the economy were leading concerns for Republican voters, Edison exit polls in California, North Carolina and Virginia showed.
Trump, who frequently denigrates migrants, has promised to mount the largest deportation effort in US history if elected.
Tuesday's results will only intensify the pressure on Haley to drop out of the race.
She did not make a public appearance on Tuesday, and her campaign has not scheduled any events going forward.
In a statement, her spokesperson said the vote showed "there remains a large bloc of Republican primary voters who are expressing deep concerns about Donald Trump".
In California, Democratic US representative Adam Schiff, the lead prosecutor in Trump's first impeachment trial, advanced to the general election to succeed the late Democratic US senator Dianne Feinstein.
His opponent will be former baseball player Steve Garvey, a Republican.
In Arizona, independent US senator Kyrsten Sinema, a former Democrat, said she would not run for re-election, setting up a battle for her seat that could determine control of the Senate in 2025.
In North Carolina, Trump-endorsed Lieutenant Governor Mark Robinson won the Republican nomination for governor.
Robinson, who would be the state's first black executive, has drawn criticism for harsh comments about LGBTQI people, women and Muslims.
He will face Democratic Attorney-General Josh Stein in what will be one of the US's most hotly contested governor races.
Haley's challenge has highlighted some of Trump's potential general election vulnerabilities.
She has reached 40 per cent in some state contests, performing particularly well among independent, well-educated and suburban voters who could play a crucial role in battleground states in November.
About one-third of North Carolina voters said Trump would not be fit to serve as president if he was convicted of a crime, while in Virginia, 53 per cent said he would be fit for the office if convicted.
Trump begins his first criminal trial on March 25 in New York, where he is charged with falsifying business records to conceal hush money payments to a porn star during his 2016 presidential run.
Trump also faces separate federal and Georgia state charges for election interference, and federal charges for retaining classified documents after leaving office.
Trump has pleaded not guilty in all four criminal cases.
Biden faces his own weaknesses, including widespread concern about his age.
