SAN ANTONIO (REUTERS) - Donald Trump swatted away the latest controversy to shadow his unorthodox march toward the Republican presidential nomination on Monday, attributing his failure to disavow support from a white supremacist to a faulty television earpiece.

On the eve of the biggest voting day in the race to pick the 2016 United States presidential candidates, the Republican front-runner tried to explain why he did not condemn backing from former Ku Klux Klan leader David Duke during a Sunday television interview.

"I'm sitting in a house in Florida with a very bad ear piece that they gave me, and you could hardly hear what he was saying, but what I heard was various groups," Trump said on NBC's Today show.

"I disavowed David Duke all weekend long, on Facebook, on Twitter, and obviously, it is never enough," the real estate billionaire added.

In a campaign that Trump has stamped with constant insults of rivals and critics and a string of controversial proposals, it was unclear whether he would be damaged by an association with white supremacist support - something that might have tanked another presidential candidacy. He has proposed a temporary ban on Muslims entering the country, called Mexican immigrants criminals and insulted women.

The former reality TV star, whose candidacy began as a long shot, has won three out of the first four state contests to pick the Republican nominee for the Nov 8 presidential election. He looks set to amass more victories when 11 states, many in the South, hold nominating contests on Super Tuesday.

Trump leads opinion polls in many of those states, as well as in most national polls where he has a double-digit advantage over closest Republican challengers Marco Rubio and Ted Cruz. His progress toward the nomination, boosted by a surprise endorsement from New Jersey Governor Chris Christie on Friday, has alarmed many in the Republican establishment.

Trump's rivals slammed him for equivocating on white supremacist support on Sunday, when he was asked repeatedly on CNN if he would condemn the Klan and disavow support from white supremacists including Duke, a former Klan grand wizard from Alabama, one of the states voting on Super Tuesday.

Rubio, a first-term US senator from Florida who is seen by some Republican leaders as the best hope to defeat Trump, said on Monday, "I don't care how bad the earpiece is - Ku Klux Klan comes through pretty clearly and he refuses to criticise it. How can someone like that be our nominee?"

Rubio has embraced some Trump's bar brawl tactics with apparent relish since a debate last week, smiling broadly when his shift to offence was greeted with claps, laughter and cheers. He has since insulted Trump's appearance and suggested he urinated in his pants during the debate.

The descent into incivility has drawn rebukes from Republicans including John Kasich, the Ohio governor who hopes to keep his own White House hopes alive beyond Super Tuesday by doing well in states that are rich in the delegates candidates need to win the nomination.

Kasich, who has prided himself on running a positive campaign, criticized his rivals for their constant name-calling in a campaign he described as "almost beyond fiction".

"It's just one controversy over another," Kasich said on CNN. "This is like every day. It's a circus."