Who's the biggest liar in the 2016 U.S. presidential contest? The Republican billionaire businessman Donald Trump or the Democratic former U.S. Secretary of State Hillary Clinton? Sen. Bernie Sanders, I-Vt., indicated his money is on Trump.

Trump called Clinton a "liar" Sunday on both NBC News' "Meet the Press" and ABC News' "This Week," while Sanders said on "This Week" that Trump is a "pathological liar."

"It's nonsense. It's just another Hillary lie," the real-estate mogul said of the former secretary of state's claim during the Democratic presidential debate Saturday night that Trump's call for Muslims to be blocked from entering the U.S. plays into the Islamic State group's hands and has been used as a recruitment tool in videos produced by the militant organization formerly known as either ISIL or ISIS. "She just made this up in thin air."

Asked whether he would change his rhetoric if he knew his words were being used as a recruitment tool by the Islamic State group, Trump replied that he would not. "My words show strength," he said.

"Much of what he says are lies or gross distortions of reality," Sanders said of Trump, adding there is no evidence to back up Trump's claim Muslims in New Jersey celebrated in the streets after the Sept. 11, 2001, terrorist attacks in New York and outside Washington.

"Nobody has seen a tape of thousands of people celebrating the destruction of the Twin Towers in New Jersey. It doesn't exist, and [Trump] keeps claiming it. That's called pathological lying," Sanders said.

On CBS News' "Face the Nation," GOP presidential contender Jeb Bush called Trump's remarks on Muslims "ridiculous," but Florida's former governor said he understands why people might gravitate toward it. "But that doesn't mean it's the right thing to do," he said.

clinton sanders Democratic U.S. presidential candidates Bernie Sanders (left) and Hillary Clinton share a laugh at the beginning of a commercial break during the debate at St. Anselm College in Manchester, New Hampshire, Dec. 19, 2015. Photo: Brian Snyder/Reuters

At the Democratic presidential debate Saturday night, Sanders said the U.S. should concentrate first on destroying the Islamic State group. Only then should the country turn its attention to the political process required to oust Syrian President Bashar Assad. In contrast, Clinton insisted both goals should be pursued in tandem.

"You can't fight ISIS and Assad at the same time," Trump said during a live telephone interview on CNN's "State of the Union," agreeing with Sanders. "I don't agree with him on much, but if he says that, I agree with him."

Trump criticized Clinton's Middle East policy, saying that although she was the most-traveled secretary of state in U.S. history, the "Middle East blew up around her." On "This Week," he called her incompetent.

Trump said the U.S. should not support any rebels in Syria. "Every time we get involved with rebels ... it ends up being far worse than the people who were there in the first place," Trump said, citing the situation in Libya where U.S. air support helped oust Moammar Gadhafi. "You have to get rid of ISIS first. Assad, we can think about later."

Also on "State of the Union," Trump rival Sen. Rand Paul of Kentucky said the possibility of having Trump in charge of the U.S. nuclear arsenal "worries me," especially because the reality-television personality showed no indication he understood what the term "nuclear triad" meant during the Republican presidential debate last Tuesday. Paul also said he thinks polls showing Trump with a commanding lead are "wildly off."

Sanders said Trump's appeal to GOP voters is that he offers "easy" solutions. "What somebody like a Donald Trump is doing is playing on the fears and anxieties of the American people," he said.

Trump said his real appeal is that voters are tired of "incompetents" running the country.