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The feud heated up on Friday, when the property mogul appeared to imply that George W. Bush was at least partly at fault for the 2001 attacks that brought down the World Trade Center, as they happened on his watch.
Pressed about whether that meant that Bush was at fault, Trump replied: "He was president, okay? Blame him or don't blame him, but he was president. The World Trade Center came down during his reign."
Jeb Bush replies on Twitter that the insinuation about his older sibling was "pathetic" and that "We were attacked & my brother kept us safe."
In a response no more than one notch above the playground, Trump tweeted, "No Jeb Bush, you're pathetic for saying nothing happened during your brother's term when the World Trade Center was attacked and came down."
@JebBush, you're pathetic for saying nothing happened during your brother's term when the World Trade Center was attacked and came down.8:29 PM - 16 Oct 2015
Trump, who retains leadership in the Republican polls, while Bush, the long-term favorite languishes in fourth place, continued to contrast himself and the former US president on Fox News Sunday.
"I am extremely, extremely tough on illegal immigration. I'm extremely tough on people coming into this country. I believe that if I were running things, I doubt those people would have been in the country," said Trump.
The prospective candidate also said that he would have ensured closer cooperation between rival government security agencies, and would have listened to those intelligence officials who had warned Bush of an imminent attack.
"I don't want Jeb Bush saying, 'My brother kept us safe,'" summed up Trump, whose fortune is estimated at $4.5 billion by Forbes, and over $10 billion by him.
Bush decided to change tack in his struggle against Trump, attacking his credibility, as opposed to trying to stay with him blow-for-blow.
Above the caption "Our national security isn't a reality TV show," Bush's team released an attack video claiming that Trump isn't fit to command the Pentagon's nuclear arsenal, and running through a catalogue of his gaffes, including his weak grasp of foreign policy, and occasional inarticulacy.
62 year-old Bush further forced the point in an interview with CNN.