The contest for Republican and Democratic contenders to become presidential candidates in the United States has entered its third round.
Polls have already opened in the Republican primary vote in South Carolina, where frontrunner Donald Trump will try to fend off Ted Cruz.
Hillary Clinton and Bernie Sanders could face a close race in the Democratic Party caucus in Nevada.
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.