George W. Bush never mentioned Donald Trump. But with his folksy touch, the former president unleashed a tough takedown Monday 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, former Florida Gov. 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 race has entered an increasingly nasty phase in South Carolina, with numerous negative ads airing on local television following an unusually caustic debate this past weekend in which Trump confronted Jeb Bush about his brother's record as president.

Trump has led polls in South Carolina, but is trying to fend off attacks from his chief rival, Texas Sen. Ted Cruz, who describes himself as "a consistent conservative" who can be trusted. Bush is trying to pull ahead of Florida Sen. Marco Rubio and Ohio Gov. John Kasich to emerge as the more mainstream alternative to Trump and Cruz, both of whom the party establishment considers unelectable.

George W. Bush 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."

With his brother as a strong warm-up act, Jeb Bush delivered an impassioned version of his campaign speech, touting his experience as Florida governor and vowing he could put Republicans back in the White House for the first time in eight years.

"I can beat Hillary Clinton," he said of the Democratic front-runner. "I can promise you that."

Appearance follows Trump's 9-11 criticisms

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.

As he praised South Carolina's Republican Gov. Nikki Haley, the daughter of Indian-born parents, Bush pointedly said, "Thank goodness our country welcomed her parents when they immigrated here in 1969."

It was a reminder of how much the Republican Party has changed since he was president. While Bush championed failed legislation that would have provided a pathway to citizenship for millions of people in the U.S. illegally, many current Republican presidential candidates have fought to outdo each other with tough enforcement policies, even mass deportations.

Jeb Bush spent months trying to figure out what role, if any, his brother might play in his campaign. 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.

But South Carolina is a state that has long been friendly to the Bush family. Presidents George W. Bush and George H.W. Bush each won two Republican primaries in the state, and their family retains deep social and political ties here.

George W. Bush has kept a low profile since leaving the White House in early 2009. He retreated to his home state of Texas, where he picked up painting and delved into work on his presidential library, public health projects in Africa, and events for wounded military service members.

The former president is the latest member of the prominent political family to hit the campaign trail to help prop up Jeb Bush. Family matriarch Barbara Bush had hit the campaign trail in New Hampshire, delighting voters with her outspoken style and tenacity, as the 90-year-old traipsed through snow to get to events.