Mr. Trump’s pointed critique of President Bush’s performance on terrorism has startled some of the conservative world’s leaders and opinion makers, like Rush Limbaugh, who likened Mr. Trump to the leftist filmmaker Michael Moore — a comparison Jeb Bush also made at the rally. Such attacks on the most recent Republican president are unheard-of among the leading Republican candidates, and it is difficult to predict how they will affect the Saturday primary in South Carolina, where the Bush presidencies, and family, are remembered fondly.

But Mr. Trump’s words may have affected the former president, who spent much of his 30-minute speech defending his administration and his personal response to the Sept. 11 attacks, even starting at the elementary school in Florida where he first learned that planes had crashed into the World Trade Center’s twin towers.

“On the way to Air Force One, from that school, Condi called me and said a plane has hit the Pentagon,” the former president said, referring to Condoleezza Rice, his national security adviser at the time. “I felt the first one was an accident, the second was an attack, and the third one was a declaration of war. I became something that no president should ever want to be — a wartime president.”

Mr. Trump’s news conference, several miles north of here, offered a striking contrast with the Bush family rally on Monday evening. Mr. Trump, in a freewheeling, sometimes stream-of-consciousness style, spoke for more than 45 minutes, criticizing — and even threatening to sue — his rivals, squinting angrily at times and dismissing questions he did not like.

Jeb Bush and his older brother, meanwhile, offered the waving, smiling feel of a reunion tour — both a look back to the Bush administration, and a look ahead to why the younger brother, whom George W. Bush introduced to cheers as “my big little brother,” is the right choice for the future of the country.

Though the tone of the rally was serious and at times somber, especially when the former president discussed the Sept. 11 attacks, he also cracked jokes. Looking older — his hairline a bit farther back, his face longer — he seemed to relish his time before the friendly crowd.