Donald Trump at a campaign rally in Las Vegas on Monday. REUTERS/Jim Young Republican presidential frontrunner Donald Trump fiercely attacked his rival Sen. Ted Cruz of Texas during a rally on the eve of the Nevada caucuses.

"This guy is sick," Trump told his supporters Monday night in Las Vegas. "There's something wrong with this guy."

He accused Cruz of running false ads against him.

"He's got an ad, something to do with I want to take away your land and I want to keep it in the federal government," Trump said incredulously. "I don't even know what the hell they're talking about."

Trump was referring to a recent Cruz ad that said Trump wanted "to keep big government in charge" of the wide swath of Nevada territory controlled by the federal government. In contrast, Cruz vowed to do everything he could to get government out of the picture.

At his rally, Trump called the ad a "scam" and then touted his support among the evangelical voter bloc that is generally considered a part of Cruz's base. Trump said the ad demonstrated why he beat Cruz in last weekend's primary election in South Carolina.

"The evangelicals didn't vote for him," Trump said. "And you know why? Because they don't like liars. They're really smart people. They don't want to vote for a liar."

Earlier in the evening, Trump repeatedly called Cruz a "liar."

"This guy Cruz lies more than any human being I've ever dealt with," Trump said. "He holds up the Bible and he lies."

Here's the Cruz ad Trump was referring to:

Trump has been attacking Cruz for weeks, calling him a liar and accusing him of "dirty tricks." The attacks have shown signs of sticking, as Cruz attempted to distance himself from the label as a new firestorm emerged Monday afternoon.

Cruz's top spokesman, Rick Tyler, shared a news story on Facebook that purported to show a third candidate, Sen. Marco Rubio of Florida, pointing to a Bible that a Cruz staffer was holding in a hotel lobby and saying that "not many answers" were in it. Rubio, in fact, said "all the answers are in there" as he gestured toward the Bible.

Despite Tyler's subsequent apology, Cruz said he asked for Tyler's resignation from the campaign. He defended Tyler as a "good man" who had made "a grave error in judgment."

But Trump said the episode was part of a pattern of Cruz's dishonesty.

"Wow, Ted Cruz falsely suggested Marco Rubio mocked the Bible and was just forced to fire his Communications Director," Trump tweeted. "More dirty tricks!"

In his tweets Monday afternoon, Trump also referred to a "phony ad" that he called "ridiculously false" and demanded that he demanded Cruz take down.