The highly-charged fight between Donald Trump and Republican presidential rival Jeb Bush over the Sept. 11, 2001 attacks raged on Wednesday morning, with Trump hitting back against a scathing op-ed Bush wrote about him in National Review. Sure, Bush and Trump have had their share of dust-ups, but this one has gotten especially heated, with both sides eager to hit the other. Why? Because each candidate appears to see a chance here to talk about issues at the core of their campaign on favorable turf.

In the back-and-forth, Bush is casting himself as a serious, prepared contender on foreign policy. And he's at his most comfortable when he is talking about policy. Contrasting himself to Trump, in Bush's view, helps him amplify his argument that he has what it takes to walk in the the White House Situation Room on Day One and comfortably make decisions.

Trump's campaign, on the other hand, is not rooted in ideology or policy contrasts. He is casting himself as a fresh, outsider alternative to his politically seasoned opponents by highlighting their vulnerabilities in a particularity blunt way. The baggage from George W. Bush's presidency has been and continues to be an issue Jeb Bush has to deal with on the campaign trail. Trump is trying to exploit it to maximum effect.

Ever since Trump suggested to Bloomberg in an interview published Friday that George W. Bush is at least partly responsible for the 9/11 attacks because “the World Trade Center came down during [his] reign," a fierce battle has ensued between the real estate mogul and the former Florida governor. Trump has posted a flurry of tweets; Bush has hit back in public remarks and released a video casting Trump as unprepared to deal with major foreign policy issues.

While Bush and Trump have sparred over several different issues before, they seem especially fired up on this issue. Bush even penned a Tuesday National Review op-ed. In it, he not only criticized Trump for echoing "the attacks of Michael Moore and the fringe Left against my brother," but pivoted to a policy-heavy pitch for his own candidacy.

"If I earn the honor of serving as Commander-in-Chief, I will reverse Barack Obama’s devastating defense cuts," writes Bush. "I will strengthen the NATO alliance and use America’s newly realized status as an energy superpower to lessen others’ dependence on Russian oil and gas, and the advantage it gives Putin. I will repair our alliance with Israel and regain the trust of our other partners in the Middle East in order to halt Iran’s aggression. And I will have a strategy to defeat ISIS and Assad and protect American interests in the region."

Trump hit back with a signature attack of his own during an interview on ABC's "Good Morning America" Wednesday, saying Bush has "had a very hard time," and he is "embarrassed by what’s happening" to him in the polls.

There are other reasons why this fight feels so nasty. It involves family. It's over an attack etched into the minds of millions of Americans. And Bush really had no choice but to defend himself on such a potent matter.

But amid that are two candidates trying to highlight what they see as some of the most convincing reasons they should be president. With the next debate just a week away, don't expect this issue to quietly fade into the background.