Donald Trump is not one to mince words, with Republicans or anyone else. 

Trump unloaded on rival Ted Cruz and former President George W. Bush in a fiery press conference in Hanahan, South Carolina, on Monday, less than one week ahead of the state's Feb. 20 Republican primary. 

“I've never seen anybody that lied as much as Ted Cruz,” Trump said after calling the Texas senator a “total basket case.” “He goes around saying he's a Christian—I don't know, you're going to have to study that.”

On Bush, Trump was equally unsparing. “The World Trade Center came down during the reign of George Bush...we weren't safe,” Trump said.

The Republican front-runner's comments came just hours before the former president was set to campaign Monday in South Carolina for his brother, former Florida Governor Jeb Bush. “Say hello to him for me. Give him my warmest regards,” Trump said with a smile.

In recent days, the Bush campaign has stepped up its attacks on Trump's temperament, saying he's unfit to be commander-in-chief.

Meanwhile, Cruz has flooded South Carolina airwaves with attack ads against Trump, seeking to portray him as an opportunistic politician who shouldn't be trusted by conservatives. In response, Trump has issued Cruz an ultimatum: apologize or risk a lawsuit on whether the Canadian-born Cruz is eligible to serve as president. 

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Trump also had a message to party leaders at the Republican National Committee. “The RNC is controlled by the establishment, and that's too bad,” Trump said, adding that RNC officials are in “violation” of a pledge the Republican candidates signed not to run as independent candidates if they lose the nomination.

Trump maintains RNC officials filled Saturday's Republican presidential debate audience with donors and lobbyists—the types of people Trump frequently attacks. In the past two Republican debates, Trump has been booed by the audience.

Earlier on Monday, Cruz released a television ad in South Carolina with a narrator saying, “Life, marriage, religious liberty, the Second Amendment—we’re just one Supreme Court Justice away from losing them all…We cannot trust Donald Trump with these serious decisions.”

Trump maintains a dominant lead in South Carolina, according to recent polls. Forty-two percent of likely Republican voters in South Carolina support the billionaire; Cruz is in second at 20 percent; and Senator Marco Rubio follows with 15 percent, according to a CBS News tracking poll conducted Feb. 10-12. None of the other candidates garnered more than 10 percent.

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