LEESBURG, Virginia --Donald Trump's challengers struggled Monday to stop him before he becomes "unstoppable," one day before Super Tuesday primaries that could cement his domination of the Republican presidential race. Trump himself strained to shout down protesters at his own rally.
Ted Cruz and Marco Rubio continued to hammer at the GOP front-runner's character and lack of policy specifics while courting voters across the South, while Trump mocked them in Radford, Virginia.
At one point, Trump was interrupted while he was talking about illegal immigration.
"Are you from Mexico?" he shouted from the stage.
Several minutes of shouting and booing ensued. The crowd hooted and Trump briefly quieted them. Then he looked in front of the stage and asked a woman, "You have a problem? Get her out."
At another point, he was interrupted by 20 or more protesters chanting "Black lives matter" and other slogans. They, too, were escorted out, shouting all the way.
Trump has won three of four early voting states, roiling a party divided over the prospect of the brash billionaire becoming its nominee.
Cruz warned that the "Trump train" could become "unstoppable" if he rolls to big victories Tuesday. Cruz cast his rival as a carbon copy of Democratic front-runner Hillary Clinton and suggested that not even Trump "knows what he would do" as president.
Florida Sen. Rubio picked up Monday where he left off over the weekend, dogging the GOP front-runner for not disavowing Sunday a white supremacist and the Ku Klux Klan.
Rubio says Trump's decision not to denounce the public support of former KKK leader David Duke disqualifies him from the nomination by "the party of Lincoln."
Trump told NBC's "Today" on Monday that he did not hear or understand the question Sunday on CNN when he was asked about Duke and the KKK, blaming a "very bad earpiece." He said that the questioner in the earlier phone-in interview had asked about Duke and various "other groups," and that he didn't want to disavow groups whose identity he didn't know. He said he had made clear his disavowal of Duke's support over the weekend in posts on Twitter and Facebook.
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.
The David Duke debate seeped into the Democratic contest, as Vermont Sen. Bernie Sanders also 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. She scored a lopsided victory in South Carolina on Saturday, fueled by a huge advantage among African-Americans, a key Democratic constituency that will also play a dominant role in several Super Tuesday states.
Clinton turned her attention to the Republican field, all-but-ignoring rival Sanders from campaign events in Massachusetts on Monday.
"What we can't let happen is the scapegoating, the flaming, the finger pointing that is going on the Republican side," she told voters gathered in Springfield, Massachusetts. "It really undermines our fabric as a nation. So, I want to do everything I can in this campaign to set us on a different course."
Trump holds commanding leads in many Super Tuesday states, 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 him.
Trump mocked the Republican establishment and his flailing rivals. "It's amazing what's going on," he told NBC, calling his campaign a "movement."