Donald Trump. AP Photo/Paul Sancya Billionaire businessman Donald Trump continued to launch attacks against Sen. Ted Cruz (R-Texas) during a Tuesday-morning interview on Fox Business.

"Ted Cruz, he's seriously — he's got a mental problem," Trump said. "Ted Cruz is a liar. He's got a problem, that he's a liar. I've never seen anybody [lie] to that extent."

Trump has repeatedly stated his disgust for Cruz in recent days, but he ratcheted up his attacks over the past weekend.

On Monday, Trump even threatened to sue Cruz over his eligibility to be president if Cruz didn't retract "lies" about Trump's record. Cruz, one of Trump's top GOP rivals, has argued that voters should be concerned about the Republican front-runner's past support for Democrats and left-leaning policies.

"Cruz is the one that does the personal attacks on me," Trump said on Fox Business. "There's never been any politician — or almost anybody in my life that I've seen — that lies so much."

Trump added:

He will say I'm against the Second Amendment. He will say I'm totally in favor of Obamacare, which is ridiculous. These are things that are staples of what I'm about. I'm totally in favor of the Second Amendment … Obamacare, I'm absolutely totally going to repeal it and replace it. And Ted Cruz says, 'Oh, he's a big fan of Obamacare.' He is a disgrace. He is a liar.

For his part, Cruz dismissed Trump's accusations as unfounded. He has sought to group Trump with Sen. Marco Rubio (R-Florida), another presidential candidate who also accuses Cruz of lying.

"It really is remarkable," Cruz said Monday when asked about Trump's lawsuit threat on Fox News.

"And it's quite odd that both Marco Rubio and Donald Trump respond the same way," Cruz added. "Which is that when anyone points to their actual records, they get very upset and begin screaming, 'Liar, liar, liar.'"