CHARLESTON, S.C. » George W. Bush never mentioned Donald Trump. But with his folky touch, the former president unleashed a tough takedown today of the billionaire businessman who has upended a Republican Party his family has long led.

“I understand Americans are angry and frustrated,” Bush said during his first campaign rally for his brother, Jeb Bush. “But we do not need somebody in the Oval Office who mirrors and inflames our anger and frustration.”

Trump’s rise has confounded the Bush family and its allies. But despite months of predicting the brash billionaire would fade, it’s Jeb Bush whose White House hopes are in peril, particularly if he’s unable to pull out a strong showing in Saturday’s South Carolina primary.

The former president emerged from his self-imposed political hibernation to try to give Bush a boost. He layered each validation of his younger brother with an implicit critique of Trump.

He urged voters to back a candidate who will be “measured and thoughtful” on the world stage. A candidate whose “humility” helps him understand what he doesn’t know. A candidate who can win in November’s general election.

“All the sloganeering and all the talk doesn’t matter if we don’t win,” Bush said. “We need somebody who can take a positive message across the country.”

The former president’s return to presidential politics has been met with blistering attacks from Trump about the unpopular Iraq war and the economic recession that began at the end of his administration. Trump has also repeatedly reminded voters that the Sept. 11, 2001, terror attacks happened on Bush’s watch.

“If the ex-president is campaigning for his brother, I think he’s probably open to great scrutiny, maybe things that haven’t been thought of in the past,” Trump told reporters Monday.

Rather than gloss over 9/11, Bush leaned in. As the crowd fell into a hushed silence, he recounted in detail his whereabouts on the morning of the attacks and praised the troops that served in the two wars he started in response.

“Your most solemn job as voters is to elect a president who understands the reality of the threats we face,” he said.

Jeb Bush has spent months trying to figure out how to handle the former president. His brother, the 43rd president, left office deeply unpopular with a nation fatigued by the Iraq War and angry over his botched response to Hurricane Katrina. He’s also a reminder to voters eager to break with the political establishment that Jeb Bush would be the third man from his family to serve as president.