Story highlights The senators are vying for support among conservative voters in South Carolina

The pair have been wrangling over immigration for months

Washington (CNN) Sens. Ted Cruz and Marco Rubio continue to feud ahead of Saturday's South Carolina primary, with Cruz pushing back against Rubio's charge that he's a liar.

"We are not in grade school where you just get to say, 'liar liar pants on fire' and not respond to the substance," Cruz said in South Carolina on Monday.

The afternoon shot at Rubio was just the latest line from Cruz, who spent his day pushing back on his colleague's accusations of lying.

"I guess Marco's team has told him, 'Well, if anyone brings up your actual record, the fact that over and over and over again you've supported amnesty, just yell liar. Just scream, you're lying, you're lying, you're lying,'" Cruz said on "Fox and Friends" on Monday. "I guess his team has decided instead you just should insult people and claim it's a lie and they're hoping to confuse the voters."

On Sunday, Rubio accused the Texas senator of false attacks over same-sex marriage, Planned Parenthood , immigration, campaign tactics and more.

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