Republican presidential hopeful Marco Rubio attacked rival Donald Trump Feb. 28, following Trump's failure to disavow an endorsement from David Duke, the former grand wizard of the Ku Klux Klan. (Marco Rubio)

ALCOA, Tenn. -- Florida Sen. Marco Rubio on Monday once again blasted Republican presidential front-runner Donald Trump for failing to denounce the Ku Klux Klan.

Trump appeared Monday on NBC's "Today" show and claimed that a "very bad earpiece" caused him to mishear a question he was asked Sunday on CNN about David Duke, the former KKK leader. Duke has endorsed Trump, saying that  voting for anyone but him "is really treason to your heritage."

Failing to denounce Duke means that Trump is "unelectable now," Rubio told supporters in an airplane hangar here.

"I don’t care how bad the earpiece is, Ku Klux Klan comes through pretty clearly – and he refuses to criticize it," Rubio said to cheers from the crowd.

"The media knows Donald Trump can’t win. And that is why they are giving him a pass," he added. "You think they don’t know all these things about his business record? You think they don’t know all of these stories? You know what they’re doing? They’re waiting ... like the hounds of hell, they will shred him to pieces" once he's the GOP nominee.

Rubio made the remarks at the first of his five stops Monday, a packed day of campaigning ahead of Super Tuesday, when 11 states will reward 595 delegates on a proportional basis.

Speaking to a crowd that filled about three-fourths of the airplane hangar, former congressman Zach Wamp began the rally by telling the crowd the GOP needs to be more inclusive because "people who are Caucasian are going to be in the minority very soon." Rubio was introduced by Sen. Lamar Alexander (R-Tenn.) and Tennessee Gov. Bill Haslam, both of whom endorsed the senator over the weekend.

Alexander said that his younger Senate colleague "can bring out the best in our country," while Haslam said he's backing Rubio because "he served in a state government" as a Florida state lawmaker.

Rubio sounded hoarse Monday morning -- he said he's starting to sound like crooner Barry White -- but he repeated personal jabs at Trump that he's been using in recent days. He suggested that a plane buzzing overhead was carrying Trump and called "Hair Force One." He reminded the crowd that Trump used undocumented immigrant laborers to build the Trump Tower. But he sprinkled his jabs with concern that Trump is on the verge of upending the Republican Party and the nation.

"On issue after issue, what he’s saying to people isn’t true," Rubio said. "It doesn’t match his record, and yet somehow it’s gotten to this point. It’s gone too far. It’s called a con job. And we’re not going to allow a con artist to take control of the party of Lincoln and Reagan."

[Spray tans, soiled pants and little hands: The GOP race goes in the gutter]

On Sunday night in Virginia, Rubio launched an especially pointed attack on Trump's physical appearance, telling supporters that the party front-runner has "small hands" and pores clogged with spray tan.

"You cannot have a policy debate with someone with no policies," Rubio told supporters in Salem, Va. "We cannot turn over the conservative movement to a con artist."


Sen. Marco Rubio (R-Fla.) greets his supporters after a rally at Patrick Henry College on Sunday in Purcellville, Va. (Ricky Carioti/ The Washington Post)

In addition to his Tennessee stop, Rubio is scheduled to appear in Atlanta's Buckhead neighborhood at midday; at the University of Central Arkansas in Conway, Ark., in the afternoon; and in Oklahoma City and Tulsa, Okla., Monday night. On Tuesday he is scheduled to hold a rally in Minneapolis before heading home to Miami for what his team is calling a "Florida Kick-Off Rally," the formal launch of his campaign to win the March 15 Florida GOP primary.