LEESBURG: Donald Trump's refusal to denounce an implicit endorsement from former Ku Klux Klan white supremacist leader David Duke has dominated the narrative just before Republican voters across 11 states head to the polls on Super Tuesday.

Trump's rivals are scrambling to stop the billionaire businessman from becoming an "unstoppable" force in the 2016 presidential contest. And even Democratic front-runner Hillary Clinton, fresh from a strong win in South Carolina, has started turning her focus on him.

Texas Sen. Ted Cruz and Florida Sen. Marco Rubio continued to criticise Trump's character and lack of policy specifics in a series of attacks Sunday while courting voters across the South, whose states dominate Tuesday's voting.

Rubio and Cruz acknowledge that time is running out to prevent the former reality television host from becoming the Republican Party's presumptive nominee, as the race to collect delegates for the party's nominating convention this summer continues.

Trump has won three of four early voting states, and Republicans are divided over the prospect of the brash billionaire becoming their nominee.

"There is no doubt that if Donald steam rolls through Super Tuesday, wins everywhere with big margins, that he may well be unstoppable," Cruz said Sunday on CBS.

Trump was asked Sunday on CNN whether he rejected support from the former KKK Grand Dragon and other white supremacists after Duke told his radio followers this week that a vote against Trump was equivalent to "treason to your heritage."

"Well, just so you understand, I don't know anything about David Duke. OK?" Trump said. "I don't know anything about what you're even talking about with white supremacy or white supremacists."

Cruz soon responded on Twitter, telling Trump: "You're better than this. We should all agree, racism is wrong, KKK is abhorrent."

Rubio went further. "We cannot be a party who refuses to condemn white supremacists and the Ku Klux Klan," Rubio told a Virginia rally. "Not only is that wrong, it makes him unelectable. How are we going to grow the party if we nominate someone who doesn't repudiate the Ku Klux Klan?"

Trump hasn't always claimed ignorance on Duke's history. In 2000, he wrote a New York Times op-ed explaining why he abandoned the possibility of running for president on the Reform Party ticket. He wrote of an "underside" and "fringe element" of the party, concluding, "I leave the Reform Party to David Duke, Pat Buchanan and Lenora Fulani. That is not company I wish to keep."

Asked about the issue on Monday, Trump told NBC he had disavowed Duke and asked, "How many times do I have to continue to disavow people."

The Duke debate seeped into the Democratic contest, as Vermont Sen. Bernie Sanders lashed out on Twitter: "America's first black president cannot and will not be succeeded by a hatemonger who refuses to condemn the KKK."

Clinton re-tweeted Sanders' message.

The former secretary of state holds a huge advantage among African-Americans, a key Democratic constituency that will play a dominant role in several Super Tuesday states.

Trump holds commanding leads across the South, with the exception of Cruz's home state of Texas, a dynamic that puts tremendous pressure on Rubio and Cruz as they try to outlast each other.

Trump mocked the Republican establishment and his flailing rivals. "It's amazing what's going on," he told NBC, calling his campaign a "movement."