Donald Trump and Jeb Bush aren't done talking about George W. Bush just yet.
The two Republican presidential hopefuls took their battle over Trump's comments about the 43rd president and the 9/11 attacks to the Sunday morning shows, with the real-estate mogul doubling down.
Trump told Fox News' Chris Wallace that he took issue with Jeb Bush's claims that "my brother kept us safe" in the aftermath of September 11th.
"Jeb said, we were safe with my brother, we were safe" Trump explained. "Well, the World Trade Center just fell down. Now, am I trying to blame him? I'm not blaming anybody. But the World Trade Center came down, so when he said we were safe, that's not safe."
When asked what he would've done during that time, Trump responded that he "would have been much different."
That difference, as Trump explained it, would be due to a toughness on border security.
"I am extremely, extremely tough on illegal immigration," Trump said. "I'm extremely tough on people coming into this country. I believe that if I were running things, I doubt those families would've — I doubt that those people would've been in the country."
Jeb Bush, on the other hand, continued to defend his brother on CNN's State of the Union Sunday morning.
"Look my brother responded to a crisis and he did it as you would hope a president would do — united the country, he organized the country and he kept us safe," he said. "There's no denying that and the great majority of Americans believe that and I don't know why he keeps bringing this up."
As Bush sees it, these and other comments from Trump, exemplify a lack of seriousness on the part of the GOP front runner and former reality TV star.
"Across the spectrum of foreign policy, Mr. Trump talks about things as though he's still on The Apprentice," Bush said.
This round of barb-trading is just the latest in what has been a long duel between Bush and Trump. Trump frequently jabs Bush on the campaign trail, complaining to audiences of Bush's "low energy" and poking fun at the former Florida governor's dropping poll numbers.
This story originally appeared on NBCNews.com