From Columbia, South Carolina -

Even though he is leading in the polls, Donald Trump was once again the most aggressive candidate in the Palmetto State on Monday, ridiculing his closest pursuers and calling Ted Cruz a "totally unstable individual" just five days before a key primary vote in this state.

"He is the single biggest liar I’ve ever come across, in politics or otherwise, and I have seen some of the best of them," Trump said of Cruz at a news conference.

ajc.com

Credit: Jamie Dupree

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Credit: Jamie Dupree

Trump again brought up the issue of Cruz's birth in Canada, and whether he is eligible to run for President.

Cruz was more than happy to reply, both on the stump and on the airwaves, as he unveiled this new ad against Trump.

"South Carolina cannot trust Donald Trump," the narrator says.

It's all part of what is becoming an increasingly testy Republican race here in the Palmetto State - the vote is Saturday.