During the second GOP debate, Republican presidential candidates Donald Trump and Jeb Bush clashed on former president George W. Bush's response to terror threats following the 9/11 attacks. (CNN)

Donald Trump’s latest assault on the GOP establishment comes in the form of harsh questions about President George W. Bush’s handling of the 9/11 attacks, forcing Jeb Bush to defend his brother’s national security record in Jeb’s campaign for the White House.

Candidate Bush has adopted a simple refrain in defense of his brother: “He kept us safe,” he has said.

But Trump’s campaign to discredit George Bush for his response to the Sept. 11 attacks and their aftermath has put Jeb Bush on the defensive at a time when Bush’s presidential campaign has been foundering.

Trump’s 9/11 criticisms began Friday and escalated Sunday when he tweeted: “Jeb, why did your brother attack and destabalize [sic] the Middle East by attacking Iraq when there were no weapons of mass destruction? Bad info?” The Trump tweet came Sunday morning, as CNN aired a pre-taped interview with Jeb Bush.

Trump raised the issue during a Friday interview with Bloomberg News, saying, “The World Trade Center came down during [George Bush’s] reign.” In an interview with The Washington Post on Saturday, Trump listed steps he said Bush should have taken.

Jeb Bush’s retort to Trump has been to suggest the real estate mogul is applying an impossible standard, one that intentionally misrepresents Bush’s remarks in defense of his brother.

“I mean, so next week, Mr. Trump is probably going to say that FDR was around when Japan attacked Pearl Harbor. It’s what you do after that matters,” Bush said during the CNN interview that aired Sunday morning. “Does anybody actually blame my brother for the attacks on 9/11? If they do, they’re totally marginalized in our society. It’s what he did afterwards that matters, and I’m proud of him.”

But in the same interview, Bush scrambled to explain why his brother shouldn’t be held to a similar standard over 9/11 as the one to which Republicans are holding Hillary Rodham Clinton over the 2012 Benghazi attack: If it happened on your watch, you’re responsible.

“Were we doing the job of protecting our embassies and our consulates, and during the period, the hours after the attack started, could they have been saved?” Bush said. “If the ambassador was asking for additional security and didn’t get it, that’s a proper point, and if it’s proven that the security was adequate compared to other embassies, fine, we’ll move on.”

Trump’s attacks align with a central Democratic criticism of George Bush’s presidency and resurrect one of the most controversial and partisan issues of the post-9/11 era.

The result may be that GOP voters rally around Jeb Bush as he defends the legacy of his brother, whose popularity has risen since he left office: A September YouGov/Huffington Post poll showed that more than 80 percent of Republicans think Bush did either a “good” or “excellent” job of keeping the country safe while president.

Republican strategists, especially those who were close to George Bush’s administration, believe Trump’s attacks will backfire.

“It is a huge miscalculation, on Donald Trump’s part, of the appetite of Republican primary voters, to believe that somehow [Bush] is to blame for 9/11,” said Katie Packer Gage, who directed advertising for George Bush’s reelection campaign. “To Republican primary voters, it’s simple: He kept us safe because he was tough, he had an immediate response."

Ari Fleischer, who was the White House press secretary when the 9/11 attacks occurred, also thinks Trump is making a mistake: “It’s very odd to run in a Republican primary and use the rhetoric of Hillary Clinton and John Kerry, but that’s what Donald Trump just did,” Fleischer said in an interview. “Donald Trump says odd things, and Donald Trump is often wrong. He’s seldom conservative, but he’s often controversial.”

Prior to the Trump assault, the “he kept us safe” refrain seemed like it could be a winning slogan. The line has earned Jeb Bush applause from debate audiences, the support of some of his competitors and the backing of a majority of Republicans, who agreed that his brother deserved credit because the country hadn’t experienced another Sept. 11.

“On Sept. 12, the CIA informed the president directly, there will be a second wave — not may, there will be,” Fleischer said. “Much of the world was hit by massive domestic terrorism by radical Islamists, but the United States wasn’t, and that’s what gave rise to this expression that ‘he kept us safe.’ ”

But Trump’s broadsides raise troubling questions for Jeb Bush, who must defend his brother’s controversial strategy in response to the attacks.

“Did he keep us safe from further foreign intervention — more war, more body bags, more bucks down the drain? No,” said Drew Ivers, an influential Republican in early-caucus state Iowa, who directed Ron Paul’s 2012 state campaign there. “Just because our boys and girls aren’t being killed on the streets of New York City, they’re still getting killed on foreign soil.”

Jeb Bush’s advisers have been considering how and whether they can use George W. in Jeb’s campaign while still maintaining Jeb’s image as his own man apart from the Bush dynasty. Though Jeb Bush’s foreign policy team includes many of his brother’s advisers, the former Florida governor has had some trouble talking about the handling of the aftermath of Sept. 11, including the Iraq war, wiretapping, and interrogation techniques many refer to as torture.

Meanwhile, Trump, who holds a significant lead over Bush for the GOP nomination in most public opinion polls, remained on the offensive.

In a Saturday interview with The Post he suggested that if Bush had a more stringent immigration policy, he could have kept the perpetrators of the 9/11 attacks out of the United States in the first place. Trump also said that Bush should not get a free pass for presiding over an intelligence community that did not communicate well, a deficiency for which the 9/11 Commission faulted the Bush administration.

“Do I blame George Bush? I only say that he was the president at the time, and you know, you could say, ‘The buck stops here,’ ” Trump said in the interview. He concluded that while “I don’t blame him . . . you always have responsibility because you’re the president.”

But Trump’s bottom line appears to be that it is hard to claim that George Bush’s policy decisions actually kept the country safe.

“I’m not sure that anybody can answer that question. Because you don’t know,” Trump said. “Was it something we did? Was it policy that kept it? All I know is that the Twin Towers came down, and I was there.”