Republican Sen. Lindsey Graham on Monday called GOP presidential front-runner Donald Trump a "Michael Moore Republican" and said he is reconsidering whether he would support the real estate mogul as the party's nominee.

During Saturday night's Republican presidential debate, Trump claimed that former President George W. Bush failed to keep the U.S. safe on 9/11, and knew Iraq did not possess weapons of mass destruction, but lied to the American people in order to invade. Both are claims that few Republicans or Democrats have ever made before, and the audience booed loudly.

Graham told MSNBC's "Morning Joe" on Monday: "So what happened in South Carolina is that we saw the Michael Moore Republican candidate emerge who is basically embracing nutjob views toward the world, and I hope this will take a toll over time because we cannot win the White House with somebody who believes that George W. Bush intentionally lied to the American people and that he was responsible for 9/11 on top of all the other things [Trump] said."

Graham, who supports former Gov. Jeb Bush but said he would back whichever candidate is nominated, told ABC News' "Good Morning America" that he's "got to really reevaluate that after what he said about George W. Bush. There's just the kooky people in the world; the mainstream Democratic party opposition to Bush did not go where Donald Trump went."

He said he has "to sit down and think about what it means to have somebody running as the nominee of the Republican Party to accuse the past president of willfully lying about the facts and circumstances of Iraq, and being responsible for 9/11."

When asked if Bush can win the South Carolina primary on Feb. 20, Graham said,  "Jeb is definitely surging; Donald Trump will not win the nomination." Graham noted to MSNBC that "very few" South Carolina Republican primary voters believe that "'W' is a liar."

Trump currently leads the polls in the state with 37 percent support, followed by Texas Sen. Ted Cruz with 17 percent support, Florida Sen. Marco Rubio with 14.3 percent, Ohio Gov. John Kasich with 10.5 percent, former Florida Gov. Jeb Bush with 10 percent and retired neurosurgeon Ben Carson with 4.5 percent, according to Real Clear Politics' average.