Donald Trump and Jeb Bush are at it again.

This time, the two GOP presidential contenders who are campaigning, in very different ways, on the future, are sparring over the past. Specifically: the legacy of the last Republican president, and whether he should be accountable for the September 11 terrorist attacks—an event traditionally considered sacred ground in politics, but not anymore in this extraordinary campaign season.

Both men are seeking to capitalize on the back and forth. For Trump, attacking Bush over his brother’s presidency has two beneficial effects: It not only drags his opponent into the past and highlights his vulnerabilities associated with the last name, but also remains true to his non-traditional, politically incorrect, anti-establishment brand. Questioning whether then-President George W. Bush did everything he could to prevent the attacks and keep Americans safe is something you might hear from Democrats, not Republicans, especially those running for the party’s presidential nomination.

Jeb Bush has seized on Trump’s attacks of his brother as a way to question the real estate mogul and reality television star’s capacity to be commander in chief. The Bush campaign is fundraising off the feud, asking supporters to donate $5 to defeat Trump and “help defend my brother.” Bush also released a web video called “Judgment” criticizing Trump’s lack of foreign policy experience while reminding viewers that presidents are in charge of the nuclear warheads.

Bush’s impassioned defense of his brother’s legacy reflects the notion that the former president is popular among Republicans. A CNN poll taken in June showed 52 percent of voters held a favorable opinion of George W. Bush. Among Republicans, 88 percent held a favorable view.

While Jeb Bush began his presidential campaign insisting that he was his “own man,” he has started to embrace his brother more. In the GOP debate, Jeb's comment that his brother’s actions after September 11 “kept us safe” was a highlight of the candidate’s performance, in the eyes of his campaign.

George W. is also, notably, joining with his parents to host a fundraiser for Jeb this weekend in Houston. The invitation is featured on the campaign website. Bush’s eagerness to take on the fight with Trump also reflects a need to improve his sliding standing in the polls and assure current supporters he is the viable alternative to Trump.

But that’s proving to be a greater challenge by the day. As the sparring escalated between the two rivals, a new NBC-Wall Street Journal found Trump leading the field with 25 percent support—his highest number yet in this poll. Carson came in just three points behind. Marco Rubio and Ted Cruz also gained ground, garnering 13 percent and 9 percent, respectively. Jeb Bush placed fifth at 8 percent.

But the most striking shift in the polling is the percentage of GOP voters who said they could not imagine supporting Bush (44 percent) compared to those who said they could not imagine backing Trump (36 percent). These numbers come after a mixed third-quarter fundraising report from the campaign.

Last month, 43 percent said they could not imagine supporting Bush, while 52 percent said they could not imagine supporting Trump.

Trump spent several weeks criticizing Jeb Bush and characterizing him as “low energy.” But when other candidates like Rubio and Carly Fiorina started to rise after well-received debate performances, Trump turned his attention from Bush to the new upstarts.

Over the weekend, however, he returned to Bush.

During an interview with Bloomberg television on Friday, Trump noted that “the World Trade Center came down during his time,” in response to Jeb Bush’s comment that his brother “kept us safe.”

Bush took to Twitter to strike back, writing: “How pathetic for @realdonaldtrump to criticize the president for 9/11. We were attacked & my brother kept us safe.”

Trump responded on Twitter: “At the debate you said your brother kept us safe-I wanted to be nice & did not mention the WTC came down during his watch, 9/11.”

Then, during an interview with Fox News Sunday, Trump again questioned the former president’s legacy: “I don’t want Jeb Bush to say my brother kept us safe because September 11th was one of the worst days in the history of this country.” Trump also asserted that his own strict immigration policies would have prevented the terrorists from entering the country.

Bush fought back during an interview with CNN’s State of the Union on Sunday. “I don't know why he keeps bringing this up,” he said about Trump’s comments. “It doesn't show that he's a serious person as it relates to being commander in chief and being the architect of a foreign policy … Mr. Trump talks about things as though he's still on The Apprentice.”

On Monday, Trump continued focusing on Bush and his brother and questioning whether the administration could have prevented the attacks.

Trump has shown over again that he won’t—and can’t, at least not yet—be deterred, no matter what he says or does. Even after he showed himself to be wobbly on foreign policy during the last debate, he has maintained a steady lead in the polls. For Bush, there appears to be a greater gamble in taking Trump to task on this particular issue, one that focuses not only on his chief liabilities but also threatens to drag the party back in time, re-litigating past policy and response, when it wants to move forward.

Meanwhile, Trump held one of his signature rallies in South Carolina Monday night, where he touted his lead in the latest poll and bashed some of his competitors, including home state Sen. Lindsey Graham. He did not, however, mention the Bush family and September 11.