Jeb Bush speaks during the Heritage Action for America presidential candidate forum in Greenville, S.C. (Chris Keane/Reuters)

Republican presidential candidate Jeb Bush thinks that a new television ad is "disgraceful" for suggesting that his brother George W. Bush didn't keep the country safe during his presidency.

Bush singled out Democratic front-runner Hillary Rodham Clinton in a tweet Sunday, suggesting that her "political machine" was behind a new 30-second ad that will be airing on cable news channels this week.

But the ad is paid for by Americans United for Change, a Democratic group founded during the second Bush administration that is not formally linked to Clinton's campaign. The message seizes on Jeb Bush's comments, made amid a verbal tussle with rival Donald Trump during last week's GOP presidential debate, as he defended his brother's national security policy and said "he kept us safe."

The comments came during a heated exchange with Trump, and Bush's campaign thinks they will contribute to the eventual demise of the Republican front-runner. But Democrats accuse Bush of either forgetting or overlooking the fact that the terrorist attacks in New York, Washington and Pennsylvania happened on his brother's watch. And polls released Sunday suggest that Trump's lead in the GOP field is holding steady.

Beyond the 9/11 attacks, the ad suggests that George W. Bush did not keep Americans safe from "a crumbling economy," "a bureaucratic catastrophe" and "an unnecessary war."

"It’s as if Jeb Bush believes his brother’s presidency began on September 12th, 2001. But we’re not letting Jeb rewrite history," Brad Woodhouse, president of Americans United for Change, said in a statement about the ad. "It’s convenient but not honest to ignore the facts that the worst attack on American soil since Pearl Harbor happened on George W. Bush’s watch after dismissing warnings from U.S. intelligence officials weeks before 9/11 that Osama bin Laden was ‘determined to strike in US’ and that his terrorist network might hijack American planes."

On Friday night, Jeb Bush reiterated his belief that his brother deftly handled national security and foreign affairs. Speaking at a Michigan conference for some of the Republican Party's most generous donors, he said that the next president will need to do a better job with international relations than President Obama has.

"I know how to do this because, yes, I am a Bush," he said. "I happened to see two really good presidents develop relationships with other countries."

[Clinton criticizes ‘failed’ effort to train Syrian rebels, says U.S. should admit at least 65,000 refugees]

For her part, Clinton doesn't completely agree with Jeb Bush's measurement of George W. Bush's presidency.

"I think it's a complicated question, because, of course, 9/11 happened," she told CBS's "Face the Nation" on Sunday. "I was a senator from New York. And I was basically consumed by my responsibility to help the people directly affected in my state and in the city.

"I do give President Bush credit for trying to bring the country together around the threats that we did face," she added. "I have said the war in Iraq was a mistake. I supported what happened in Afghanistan. So, if you sort it all out, you know, it's a mixed — it's a mixed picture."

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