Donald Trump and Jeb Bush were at it again.

Trump upped the ante in criticizing Jeb Bush by slamming his brother George W.'s presidency and at least partially blaming the elder Bush brother for Sept. 11.

"When you talk about George Bush, I mean, say what you want, the World Trade Center came down during his time," he told Bloomberg. When the questioner said he couldn't blame Bush for terrorist attacks, Trump responded this way: "He was president, OK? ... Blame him, or don't blame him, but he was president. The World Trade Center came down during his reign."

And then, it all blew up on Twitter.

Jeb Bush lashed out, calling Trump "pathetic."

Trump shot back with a version of the elementary school barb, "I know you are, but what am I?"

"No... you're pathetic," Trump tweeted.

Trump was asked about his remarks Friday night. CNN described the scene this way:

When asked by CNN after a rally at a local high school here if he thought the attacks were George W. Bush's fault, Trump, after pausing to listen to the question, walked away.

The exchange highlighted the ongoing feud between the two Republican candidates vying for the GOP nomination. But both have big vulnerabilities and hurdles in their way. Trump has the enthusiasm and support right now, but lacks the campaign infrastructure to go the distance. Bush has the infrastructure, but is lacking in support.

Saturday marks the 90th straight day that Trump has led in the GOP primary, according to the Real Clear Politics average of national polls. That's three months in which Trump has been the consistent leader in the Republican race to be president.

What's more (and more important than national polls), he's held the lead by himself in all three early primary states for more than two months: July 28 in New Hampshire, Aug. 2 in South Carolina and Aug. 4 in Iowa.

The third-quarter fundraising numbers exposed a nagging Trump vulnerability — whether he's serious about winning the nomination.

He's making no effort to raise money, and despite his claims of being really rich ($10 billion!), he only contributed about $100,000 to his campaign between July and the end of September. (Overall, he's given himself about $3 million.)

He isn't running TV ads and is instead relying on free media. He hasn't hired a professional pollster (yet cites every public poll he can get his hands on — whether they're well conducted or not) and his campaign doesn't regularly collect names for organizing email lists at events (although Trump is starting to show signs of trying to build out a real campaign infrastructure).

Despite all that, he still leads in the polls. But that very well may turn out to be a blind spot for him. Plenty of others who led at one point or another have faltered in past cycles. Trump's message — and political incorrectness — is clearly resonating with GOP primary voters, but he's not building the kind of infrastructure it usually takes to win.

Of course, maybe the traditional rules won't apply this year. Rules are written to be re-written, but Trump has missed opportunities to supercharge his candidacy.

The irony here is that Bush has built the infrastructure, but doesn't have the numbers. His last name is a vulnerability with many voters, as evidenced by his low favorability ratings in national polls (and the replies to Bush's tweet defending his brother).

Still, others don't think he lives up to the family name held by two former presidents. "I like the name," one Iowa Republican told NPR's Don Gonyea earlier this month. "I just don't feel he backs it up the way the other two did."

That's quite the pickle.

And, of course, neither of Bush nor Trump might be the nominee. While these two continue to bicker, other candidates, like Marco Rubio or Ben Carson or John Kasich or Carly Fiorina or someone else, are hoping to shoot the gap.

Update at 11:45 a.m.

Copyright 2016 NPR. To see more, visit http://www.npr.org/.