Donald Trump and Hillary Clinton secured vital wins in the race for the White House, as Jeb Bush quit after a disappointing result.
Billionaire tycoon Mr Trump widened his lead over the Republican party's presidential field claiming a big victory in South Carolina as the contest moved into the south.
Mr Bush ended his quest to follow his father and brother to the White House, suspending his campaign after a fourth place finish.
Meanwhile, out west, Mrs Clinton beat Vermont senator Bernie Sanders for a crucial win in Nevada's Democratic caucuses.
Mrs Clinton and Mr Trump's victories put them in strong positions as the 2016 presidential election heads towards Super Tuesday - the multi-state voting contests on March 1.
"There's nothing easy about running for president," Mr Trump said at his victory rally. "It's tough, it's nasty, it's mean, it's vicious. It's beautiful - when you win it's beautiful."
Mrs Clinton's roughly five-point win eased the rising anxieties of her backers, who feared a growing challenge from Mr Sanders.
Mr Trump's strong showing in South Carolina marked his second successive victory in the Republican primaries and strengthened his unexpected claim on the party nomination.
No Republican in recent times has won New Hampshire and South Carolina and then failed to win the nomination.
Senators Ted Cruz and Marco Rubio were locked in a race for second place in South Carolina. Mr Bush and other candidates lagged far behind.
"This has become a three-person race," Mr Rubio said of his strong finish, which with Mr Bush quitting bolsters his case that he is the candidate of mainstream Republicans.
Mr Cruz, who has run as a political outsider, harked back to his win in the lead-off Iowa caucuses as a sign he was best positioned to take down Mr Trump.
He urged conservatives to rally around his campaign, saying pointedly: "We are the only candidate who has beaten and can beat Donald Trump."
For both parties, the 2016 election has laid bare voters' frustration with Washington and the influence of big money in the political system.
The public mood has upended the usual political order. That gave Mr Sanders, who put up a stiff challenge to Mrs Clinton in Nevada, and Mr Trump openings over many more mainstream candidates.
In Nevada, Mrs Clinton won the backing of voters who said electability and experience were important in their vote. But in a continuing sign of her vulnerability, Mr Sanders did best with voters looking for a candidate who is caring and honest.
She capitalised on a more diverse Democratic electorate who helped her rebound after a second-place finish to Mr Sanders in the New Hampshire primary.
"Some may have doubted us, but we never doubted each other," Mrs Clinton told her cheering supporters during a victory rally in Las Vegas. "This one is for you."
She said Americans are "right to be angry," but are also hungry for "real solutions".
Mr Sanders, a self-described democratic socialist, congratulated Mrs Clinton on her victory, but then declared his campaign has "the wind at our backs as we head toward Super Tuesday".
Mrs Clinton now leads in delegates pledged to her at the Democratic Party's national convention in July, but only has a fraction of the number needed to secure the nomination.
Her win means she will pick up at least 18 of Nevada's 35 delegates, while Mr Trump is also accumulating a delegate lead among Republicans.
No candidate has shaken the establishment more than Mr Trump. He spent the week threatening one rival with a lawsuit, accusing former president George W Bush of lying and even rowing with Pope Francis on immigration.
The Trump win drove Mr Bush, once the frontrunner, who was counting on his family's broad popularity in South Carolina, out of the race. Mr Bush is the son and brother of former presidents.
Now, the Trump victory foreshadows a solid performance in the collection of Southern states that vote on March 1.
Victories in those Super Tuesday contests could put the billionaire in a commanding position in the delegate count, which decides the nomination at the party's national convention in July.