As Republican presidential contenders Donald Trump and Jeb Bush feud over comments Trump made regarding former President George Bush's leadership during the 9/11 terrorist attacks, fellow presidential candidate Sen. Lindsay Graham seems to want in on the action.

In a series of tweets Saturday, the South Carolina senator slammed Trump as "not ready to be commander-in-chief" and called on the real estate mogul to apologize to the former president. Trump ignited the debate Friday when he suggested that the 43rd president was in part to blame for the 9/11 terrorist attacks as "the World Trade Center came down during his reign."

.@realDonaldTrump suggesting George W. Bush is a loser b/c we were attacked shows that you don't understand this war.

— Lindsey Graham (@LindseyGrahamSC) October 17, 2015

.@realDonaldTrump If you don't understand that we need to hit them before they come here, you're not ready to be Commander-in-Chief

— Lindsey Graham (@LindseyGrahamSC) October 17, 2015

.@realDonaldTrump should apologize to George W. Bush who understood the evil nature of this enemy.

— Lindsey Graham (@LindseyGrahamSC)

October 17, 2015

Former Florida Gov. Bush called Trump's comments "pathetic" in a tweet to which Trump shot back by calling him "pathetic" instead.

"Like it or not, our country needs more energy and spirit than you can provide!" Trump tweeted.

Trump is leading the vast Republican presidential field with Bush coming in fifth, according to a recent CBS News poll. Graham did not register on the poll. A Public Policy Polling report in September found that 78 percent of South Carolina Republican primary voters wished the hawkish Republican would drop out of the race.