Charleston - The race to the White House entered its third round Saturday, with Republicans voting in their first southern primary in South Carolina and Democrats choosing between Hillary Clinton and Bernie
Voters streamed to the polls in South Carolina after they opened at 07:00, in what is likely to be an important test of strength for frontrunner Donald Trump.
Pre-vote surveys showed the billionaire businessman with a big lead over five Republican rivals in the Palmetto State.
Tim Nielson, 56, emerged from a voting place in Mouth Pleasant, South Carolina wearing a red, white and blue "I voted" sticker.
He said he had switched from Democrat to Republican to vote for Trump. If Trump wins the White House, said Nielson, "maybe he'll change some ways."
Trump is banking on a big symbolic win ahead of "Super Tuesday" - March 1, when about a dozen states will go to the polls to choose candidates for the November 8 presidential election, with a quarter of the nominating delegates up for grabs.
"It's crunch time, folks," Trump, 69, told voters at a North Charleston rally, his final pitch Friday before the primary.
He finished second to Texas Senator Ted Cruz in Iowa on February 1, but secured a commanding win in New Hampshire one week later.
The onetime reality TV star, who has upended the political landscape with his brash style and controversial comments, has his eye on a particular date: March 15.
After that day, many of the Republican primaries will be winner-takes-all in terms of delegates. If his five rivals are still in the race at that point, they will be splitting the anti-Trump vote - and increasing his chances of winning the nomination.
On the eve of the primary, Trump led with about 28% of likely Republicans voters backing him, according to an NBC-Wall Street Journal poll.
The ultra-conservative Cruz followed with 23%. Trailing were Senator Marco Rubio at 15 percent and former Florida governor Jeb Bush at 13%.
Rubio and Bush are under intense pressure to fare well 0n Saturday, as is Ohio Governor John Kasich and retired neurosurgeon Ben Carson, whose campaign has struggled to gain ground.
Bush, who has enlisted his famous family to help his flagging cause, visited a voting station in Greenville, South Carolina with his 90-year-old mother Barbara.
Carson, for his part, appealed to voters in a Fox News interview not to let the election become a choice between "the lesser of two evils."
"You don't understand South Carolina politics. It's horrible," said Edwin Pearlstine, a retired beer distributor and brewery owner who voted for Kasich even while conceding the Ohio governor has little chance of winning.
If Trump wins the Republican nomination, he said, "I got a pretty place right on the beach in the Bahamas. I'll just go stay there."
In the past week Trump repeatedly called Cruz a liar, and his attorneys even sent the senator a cease-and-desist letter over a TV ad that uses a 1999 interview in which Trump said he was "very pro-choice" on abortion rights.
Trump has since changed his stance on the sensitive issue.
To the west, in the desert state of Nevada, Democrats will caucus - group themselves together by candidate to voice their support - starting at 11:00 (19:00 GMT).
Both Clinton and Sanders have been working hard to reach out to the African-Americans, Hispanics and Asian-Americans that make up roughly half of the state's population.
Clinton, who won by a hair in Iowa but was crushed by Sanders in New Hampshire, is counting on a major Hispanic voter turnout, especially among Las Vegas hotel and casino employees.
Nevada has some three million residents, and the population is overwhelmingly concentrated in two large urban centers, Las Vegas and Reno.
Since Wednesday, the former secretary of state has visited staff at casinos in Las Vegas, where workers can "caucus" right on the famous Strip.
Clinton says she is the natural Latino ally on immigration, and if elected she promises a quick path to citizenship for those in the country illegally.
The former first lady, 68, has relentlessly attacked Sanders for voting against immigration reform in 2007. Sanders, however, said the measure gave little protection for foreign "guest workers," and that he voted for a 2013 immigration reform bill that died due to Republican opposition.
"As president, I will do everything that I can to pass immigration reform and a path toward citizenship for those who today are undocumented," Sanders, 74, said Thursday on MSNBC.
Clinton claims that Sanders is offering impractical, pie-in-the-sky ideas.
But Sanders's camp is convinced that young minority voters will back him.
Sanders, a self-described democratic socialist, is hoping that his own civil rights activist background will attract young voters.
The Chicago Tribune on Friday unearthed a 1963 photo of a 21-year-old Sanders being hauled away by police during a protest against Chicago school segregation.
"Bernie identified it himself," campaign adviser Tad Devine told the Tribune. At the time Sanders was charged with resisting arrest, found guilty and fined $25.