Donald Trump told South Carolina voters Tuesday to be wary of aircraft giant Boeing opening a plane finishing plant in China, saying he would not let it happen if he were president.

"Be careful" because Beijing is "making Boeing build this massive plant in China" in order to secure orders, Trump warned.

Should Boeing, which has a major plant in South Carolina, launch operations in the Asian giant as it announced last year that it would, it's "bye-bye to South Carolina," he told an exuberant crowd at a rally in North Augusta.

"It won't happen if I'm president, by the way."

Trump played up his international business acumen at a rally in North Augusta ahead of Saturday's Republican primary, the third statewide contest in the long road to the nomination.

But he and his rivals, including Ted Cruz, Marco Rubio and Jeb Bush, also traded political punches and laid out attack lines ahead of the crucial primary, the first in the US South.

First-term Senator Rubio, mindful of the large evangelical population in the state, told a town hall in Beaufort early Tuesday that his faith would help guide his decisions in the Oval Office.

Cruz, another freshman US senator, sought to boost his support among military veterans in South Carolina, taking to the decks of the famed decommissioned World War II battleship USS Yorktown to declare he would restore US military might if elected, after "years of neglect by President Barack Obama.

"Starting next year our sailors won't be on their knees with their hands on their heads," Cruz said referring to the American sailors held in Iranian custody last month after their ship entered that country's waters.

"Instead, they will be standing on the decks of the mightiest ships the world has ever known with their heads held high, confident that the great country that they volunteered to serve has their back."

Trump meanwhile leads handily in South Carolina, with a new CNN poll showing him leading with 38 percent over arch-conservative Cruz, who has 22 percent support.

The 16-point spread is down from the 20-point RealClearPolitics average, suggesting a softening in some support after Saturday's bruising debate slugfest.

"Why aren't they beating me?" Trump taunted, to loud cheers in North Augusta.

Even with his commanding lead, he was calling on supporters to crowd the polls.

"Assume that we're behind, because you've gotta go vote," Trump said.

Supporter Jennifer Twilley, reflecting on the anger and frustration coursing through the US electorate, said Trump's status as a business magnate and political outsider made him ideal for the job.

"I don't really want an established politician anymore," Twilley, an engineer with General Dynamics who was wearing a pink button that read "Hot Chicks for Donald Trump," told AFP.

She dismissed the concerns about his braggadocio and confrontational rhetoric.

"I don't care about the drama, I just want him to fix the financial problems," she said.

More than halfway through his hourlong speech, a protester interrupted Trump but was shouted down by supporters. She was escorted out of the venue with her middle fingers raised in the air.

Bush, a former Florida governor, was aiming to capitalize on the wave of publicity when his brother, the former president George W. Bush, hit the campaign trail with Jeb for the first time.

Seeking to present himself as the candidate most capable of rebuilding the military, Bush visited a gun manufacturer in state capital Columbia, where he spoke to company employees and walked away with his own personally engraved hand gun.

He tweeted a photograph of the firearm afterwards, with a single word of commentary: "America."