It started all over comments Trump made that appeared to criticize George W. Bush for his involvement in 9/11.
On Fox News last night, Bush took to the airwaves to slam Trump for his lack of policy credentials.
"Donald Trump is not a serious candidate as it relates to foreign policy,” he said, adding that he doesn’t believe Trump has the capability of creating strategy overseas.
Over the weekend, Bush had taken Trump’s comments and used them to fundraise. Part of his email to supporters read:
"Our national security is not a reality TV show. Donald Trump lacks the judgment on national security issues required to lead our country at a time of growing turmoil. Please stand with me in defending my brother from Donald Trump’s pathetic attacks."
Trump, for his part, had engaged as is his way; on Twitter.
Speaking to Fox News Monday, Trump continued his pledge that his immigration policy would have stopped the 9/11 terrorists going on to say his plan would have a strong "whistleblower” system.
“My standards were much higher for Visas if you look at what I'm doing,” he said. "My standards are much higher for Visas, number one. Number two, we'd have a massive whistleblower system which you have to have. It's hard to believe you had 18 or 19 people and nobody, nobody knew what they were doing. "
Bloomberg’s Stephanie Ruhle asked if Trump could reassure Americans in times of national emergencies, as the 43rd president did after Sept. 11.
“He was president, OK? Don’t blame him or don’t blame him, but he was president," Trump continued. "The World Trade Center came down during his reign. If you look at Sandy Hook, those people are still begging for help."
Bush quickly defended his brother on Twitter.
This particular fight is one that is long-standing between the two rivals.
Bush has been a heavy establishment favorite to win the nomination, but he’s been struggling in the polls, often tripled in support by the brash frontrunner who leads the GOP pack.
“Your brother and your brother's administration gave us Barack Obama because it was such a disaster those last three months,” Trump said.
Bush quickly struck back. “You know what? As it relates to my brother, there is one thing I know for sure, he kept us safe,” he said.
On the campaign trail, Trump frequently chastises Bush for being “low-energy”, while Bush’s preferred method of attack is pointing out the real estate mogul’s lack of policy chops.