Five states hold contests on Saturday, the first test of voter sentiment since a public rupture within the Republican Party pitted supporters of real estate mogul Donald Trump against a collection of establishment figures trying to block his path to the 2016 United States presidential nomination.

The Republican primary in Louisiana and caucuses in Kentucky, Kansas, and Maine have overshadowed the Democratic race, in which front-runner Hillary Clinton looks to maintain her lead in delegates over Senator Bernie Sanders with a primary in Louisiana and caucuses in Kansas and Nebraska. On Sunday, Democrats have caucuses in Maine and Republicans vote in Puerto Rico.

Donald Trump has issued a warning to those in the Republican Party plotting a way to stall his candidacy. Credit:AP

The simmering Republican civil war boiled over on March 3 when Mitt Romney, the party's 2012 presidential candidate, called Mr Trump a vulgar bully unfit to be president, and said his policies would drive the US into recession. Mr Romney urged Republicans to vote strategically in upcoming nominating contests as way to deny Mr Trump the 1237 delegates needed for the nomination. If successful, that may force a contested convention in Cleveland in July, where delegates could pick someone else to be the party standard-bearer in the November general election.

Mr Trump, who has won 10 of the first 15 nominating contests, has brushed off such criticism while emphasising how he's attracting new voters to the Republican Party with his brash, "tell it like it is" style.