LEESBURG, Va. — The final-days sprint to Super Tuesday has erupted into a feud over a white supremacist as Donald Trump’s Republican rivals scramble to stop the billionaire businessman from becoming an “unstoppable” force in the 2016 presidential contest.

Ted Cruz and Marco Rubio continued to hammer the GOP front-runner’s character and lack of policy specifics in a series of attacks Sunday while courting voters across the South. But it was Trump’s refusal to denounce an implicit endorsement from former Ku Klux Klan leader David Duke that dominated the narrative less than two days before Republican voters across 11 states head to the polls.

The new focus comes as Trump’s rivals acknowledge that time is running out to prevent the former reality television host from becoming the Republican Party’s presumptive nominee. He took a new step in that direction by earning the endorsement of Alabama Sen. Jeff Sessions, a would-be Cruz ally who backed Trump instead.

“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’ “Face the Nation.”

Trump was asked Sunday on CNN’s “State of the Union” 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 told host Jake Tapper. “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 in a message to thousands of supporters in Leesburg, Virginia: “We cannot be a party who refuses to condemn white supremacists and the Ku Klux Klan,” Rubio said. “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 was asked Friday by journalists how he felt about Duke’s support. He said he didn’t know anything about it and curtly said: “All right, I disavow, ok?”

He 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.”