LEESBURG, Va. » Republican front-runner Donald Trump drew sharp criticism from his rivals in both parties today for refusing to denounce an implicit endorsement from former Ku Klux Klan leader David Duke, raising the specter of racism as the presidential campaign hits the South.

Trump was asked 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 said. “I don’t know anything about what you’re even talking about with white supremacy or white supremacists.”

Trump’s comments came the same day he retweeted a quote from Benito Mussolini, the 20th century fascist dictator of Italy. And in a boost for his campaign in the South, he scored the endorsement of Alabama Sen. Jeff Sessions, one of the most strident opponents of immigration reform on Capitol Hill.

But it was Trump’s statements about Duke that sparked a wave of censures with just two days to go before 11 states hold GOP primaries involving about a quarter of the party’s total nominating delegate count. Several states in the South, a region with a fraught racial history, are among those voting in the Super Tuesday contests.

Marco Rubio quickly pounced on Trump’s comments, saying the GOP “cannot be a party who refuses to condemn white supremacists and the Ku Klux Klan.”

“Not only is that wrong, it makes him unelectable,” Rubio told thousands of supporters gathered in Leesburg, Virginia. “How are we going to grow the party if we nominate someone who doesn’t repudiate the Ku Klux Klan?”

Texas Sen. Ted Cruz called Trump’s comments “Really sad.”

“You’re better than this,” Cruz wrote on Twitter. “We should all agree, racism is wrong, KKK is abhorrent.”

Trump has won three of four early voting states, roiling a party divided over the prospect of the brash billionaire becoming its nominee. Late Sunday, Nebraska’s Ben Sasse became the first sitting Republican senator to say explicitly that he would not back Trump if he does win the nomination.

“If Trump becomes the Republican nominee my expectation is that I’ll look for some 3rd candidate — a conservative option, a constitutionalist,” Sasse wrote on Twitter.

With a strong showing on Super Tuesday, Trump could begin to pull away from his rivals in the all-important delegate count.

In the Southern states that vote Tuesday, Republican candidates will face an electorate that is overwhelmingly white. In South Carolina, the only Southern state to have voted so far, 96 percent of the GOP primary electorate was white, while 6 in 10 voters in the Democratic race were black.

While the South was once a Democratic stronghold, many white conservatives who backed the party started moving toward the GOP during the civil rights movement. Trump has borrowed from the rhetoric former President Richard Nixon used during that time to appeal to working-class white voters, describing his campaign has a movement of the “silent majority.”

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 and derail the real estate mogul.

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?”

The billionaire 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.”

Democrat Bernie Sanders also lashed out at his Republican rival on Twitter, writing: “America’s first black president cannot and will not be succeeded by a hatemonger who refuses to condemn the KKK.”

Trump also garnered backlash for retweeting a quote from Mussolini, which read: “It is better to live one day as a lion than 100 years as a sheep.”

Trump told NBC’s “Meet the Press” on Sunday, “I know who said it. But what difference does it make whether it’s Mussolini or somebody else? It’s certainly a very interesting quote.”

Rubio and Cruz, two first-term senators, continued a personal and policy-based barrage against Trump, warning his nomination would be catastrophic for the party in November and beyond.

“We’re about to lose the conservative movement to someone who’s not a conservative and (lose) the party of Lincoln and Reagan to a con artist,” Rubio said Sunday on Fox News.

On the Democratic side, Hillary Clinton, who received another burst of momentum Saturday after her lopsided victory in South Carolina, turned her attention to the Republican field on Sunday, all-but-ignoring rival Bernie Sanders during campaign events in Tennessee.

Starting her morning with stops at two Memphis churches, Clinton offered an implicit critique of Trump, issuing a call to unite the nation and asking worshippers to reject “the demagoguery, the prejudice, the paranoia.”

Asked by actor Tony Goldwyn, who later campaigned with Clinton in Nashville, about her thoughts on Duke’s support for Trump, Clinton described it, simply, as “pathetic.”

Trump also rejected calls from Rubio — who he repeatedly referred to Sunday as “Little Marco” — and Cruz to release his tax returns, saying he can’t share returns that are under IRS audit. The senators on Saturday released summary pages of several years’ worth of their personal returns. Trump says he’s already shared his personal financial details in separate disclosure forms.

Separately, Cruz warned the “Trump train” could become “unstoppable” if he rolls to big victories Tuesday. Cruz cast Trump as a carbon copy Clinton and suggested that not even Trump “knows what he would do” as president.

Still, Cruz confirmed to CNN’s Jake Tapper that he “will support the Republican nominee, period, the end.” Rubio has sidestepped questions about whether he could support Trump.

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Associated Press reporters Laurie Kellman and Julie Pace in Washington contributed to this report.