The contest for Republican and Democratic contenders to become presidential candidates in the United States has entered its third round.
The Republican primary is in South Carolina, where frontrunner Donald Trump is trying to fend off Ted Cruz.
In the Democratic Party caucus in Nevada, early indications of voting suggested Hillary Clinton was leading Bernie Sanders in a close race.
The votes could be key ahead of the “Super Tuesday” round on 1 March.
On that day, about a dozen states will choose their candidates for the 8 November presidential election, with about a quarter of all nominating delegates up for grabs.
Before that, the two parties swap around in these two states – Republicans holding a Nevada caucus on Tuesday and the Democrats a South Carolina primary on 27 February.
These rounds could be crucial in particular for Republican candidates who have been lagging behind the leading pair.
Polling in South Carolina is scheduled to end at 19:00 local time (00:00 GMT).
The leading Republican pair split the first two rounds – Mr Trump winning in New Hampshire, a week after Mr Cruz had triumphed in Iowa.
Mr Trump has continued his colourful campaign this week with a public spat with Pope Francis on Christian values.
At a rally in North Charleston on Friday, he told supporters: “It’s crunch time, folks.”
Referring to his self-funded campaign, he added: “I don’t want your money. We want your vote.”
He called for a “big mandate”, aware that a win in South Carolina could propel him to victories in important southern states that vote on Super Tuesday.
Mr Cruz has enlisted an army of volunteers to try to haul back Mr Trump’s lead in the state – put at about 28% to 23% in an NBC-Wall Street Journal poll of Republican voters.
That poll suggested Senator Marco Rubio was third on 15%, followed by Jeb Bush on 13%.
Both are under massive pressure to generate support for their campaigns. Whoever loses this personal battle could face tough questions about the future of their candidacy.
Mr Bush turned his fire on Mr Trump. He said at a polling place in Greenville, South Carolina: “Trump can’t win, plain and simple. He can’t be president. A ton of people would be very uncomfortable with his divisive language and with his inexperience.”
The lagging John Kasich and Ben Carson face even tougher fights to rouse support.
In Nevada, Democrats gathered at about 200 caucus meetings across the state to show their support for Mrs Clinton or Mr Sanders. The sites were scheduled to close at 15:00 local time (20:00 GMT).
Early polling of voters suggested strong support among young voters for Mr Sanders but a healthy lead in the women’s and Hispanic vote for Mrs Clinton.
Fox News said that, with 46% of votes in, Mrs Clinton had an overall lead of 51.8% to Mr Sanders’ 48.1%.
The pair also split the first two rounds – Mrs Clinton winning narrowly in Iowa, before being soundly beaten in New Hampshire.
Nevada represents the most racially diverse battleground so far, though, with both candidates trying to court the vote of African-Americans, Hispanics and Asian Americans, who make up about 50% of the state’s population.
Sanders benefits from disillusionment with Obama – Anthony Zurcher, BBC News, Nevada
Veterans of the first Obama campaign in 2008 painted a picture of an inspirational candidate who had gone astray once in power.
“Where he failed is not sustaining that movement. He just dropped the ball on that,” said Debra Mayes of Los Angeles.
Sanders supporters focused on what they see as the shortcomings of the Obama administration, and of moderate Democrats beholden to their corporate donors.
The establishment Democrats, they said, essentially wasted the majorities the party had built during the years leading to Mr Obama’s sweeping victory.
If Democratic leaders, including Mr Obama, had been more ambitious, they would have been able to establish a lasting congressional majority, the argument goes.
Mrs Clinton had hoped to use the strength of her support among minority communities to pull away from Mr Sanders.
She has accused him of “pie in the sky” ideas on immigration reform.
Mr Sanders’ socialist message has generally energised the youth vote and a win here, or even a narrow loss, could be a huge boost ahead of Super Tuesday.