Punches are flying ahead of the biggest prize so far in the US presidential nominating process as voters in a dozen states prepare to weigh in on Super Tuesday.

Republican presidential contenders Marco Rubio and Ted Cruz escalated their fight against front-runner and billionaire businessman Donald Trump on Friday after taking turns taking aim at Trump in the final debate before the crucial primaries a day earlier.

Among Republicans, Trump leads in national polls and is on top in most of the states that will vote next week on so-called Super Tuesday, which could give him the necessary momentum to become the Republican Party's candidate.

"It's time to pull (Trump's) mask off so people can see what we're dealing with here. What we're dealing with here is a con artist," Rubio told supporters in Texas.

Cruz dismissed Trump as a liberal masquerading as a Republican.

"Do you want to go with Donald Trump and have the election in November be two rich New York liberals?" he asked, referring to former secretary of state and Democratic front-runner Hillary Rodham Clinton, who hopes to maintain momentum as she faces a formidable challenger in Senator Bernie Sanders.

Trump snagged an endorsement Friday from New Jersey Governor Chris Christie, who ended his own presidential bid earlier this month after a poor showing in New Hampshire.

At a surprise announcement alongside Trump in Fort Worth, Texas, Christie declared Trump "is rewriting the playbook."

Christie sought to portray Trump as the best positioned candidate to go after Democratic front-runner Clinton in November elections, even as Rubio and Cruz claimed they were in the best position to do so.

Trump supporters like George Bryant, a 65-year-old small business owner in Manassas, Virginia, seem unlikely to be swayed by Trump's challengers.

"We need somebody who's not a politician to get in there," he told dpa as he ate lunch with a co-worker. "He makes more sense than anyone I've seen."

Both of Trump's chief opponents, Rubio and Cruz, hope to emerge as a clear alternative to the frontrunner.

"What you're going to do on Tuesday is not a poll, it counts," Rubio said.

Some 600 delegates, or about a quarter of those needed to secure the Republican nomination, are at stake, while about 1,000 are up for grabs among Democrats, some 20 per cent of the total.

Among Republicans, Texas is the biggest prize with some 155 delegates, followed by southern states Georgia, Tennessee and Alabama.

Cruz was favoured in his home state of Texas, but some opinion polls put Trump in a virtual tie with the Lone Star senator. The state is considered a must-win for Cruz, who also hopes to hold off a challenge from Rubio.

"I believe the voters on Super Tuesday are not looking for another Washington deal maker," Cruz told reporters in Tennessee. "What the voters are looking for is a principled conservative."

Among Democrats, Clinton hopes on Super Tuesday to again take control of a race that was once considered smooth sailing for her before Sanders nearly tied with her in Iowa and then defeated her by a large margin in New Hampshire.

Clinton and Sanders first face off in a primary Saturday in South Carolina, where Republicans held their contest last week.

Clinton's campaign had referred to her "firewall" against Sanders in many of the southern states that vote Tuesday, where large African American populations have long favoured Clinton. Clinton has also secured the endorsement of several prominent African American lawmakers and leaders.

Sanders however has also sought to gain inroads in the community, and one African American supporter dismissed the idea that Clinton should automatically receive support from black voters.

"I'm not her firewall," Debra Thompson, 63, told dpa at the Sanders rally. "She hasn't done one thing for the African American community."

Clinton holds strong double-digit leads in most Super Tuesday states, but Sanders has a large advantage in his home state of Vermont as well as in neighbouring Massachusetts.

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