CHARLESTON, S.C., Feb 15 (Reuters) - Former President George W. Bush came to the aid of his brother Jeb Bush's Republican presidential campaign in South Carolina on Monday with a rousing endorsement of his character and a call for voters to reject the angry bluster of Donald Trump.

The appearance of the elder Bush on the campaign trail may help Jeb Bush with South Carolina Republicans who hold the former president in high regard. But it also carries some risks, given his launching of the Iraq war in 2003, which ended up being unpopular with many Americans and which Republican front-runner Trump has seized on to criticize him.

George W. Bush, who has stayed out of politics for the most part since leaving office in early 2009, showed he remains an engaging speaker, generating cheers repeatedly over 20 minutes from the biggest crowd Jeb Bush has enjoyed in his campaign.

Without mentioning Trump by name, the 69-year-old George W. Bush left no doubt he was talking about the New York billionaire who uses incendiary rhetoric at his campaign events.

"These are tough times and I know that Americans are angry, but we do not need someone in the Oval Office who mirrors and inflames our anger and our frustrations," the former president said.

Real strength, he said, means facing challenges and prevailing.

"Strength is not empty rhetoric. It is not bluster. It is not theatrics. Real strength comes from integrity and character. And in my experience, the strongest person isn't usually the loudest person in the room," George W. Bush said.

Whether the elder Bush's presence will help his 63-year-old brother in the South Carolina Republican primary on Saturday remains to be seen. Jeb is running fourth in polls in South Carolina, behind Trump, Texas Senator Ted Cruz and Florida Senator Marco Rubio.

Trump, at a news conference in Charleston earlier in the day, continued a stream of insults directed at the Bush family, insisting that the former president bore responsibility for the Sept. 11, 2001, attacks that took place on his watch.

"Excuse me, the World Trade Center came down during the reign of George Bush, right? It came down. That was the greatest attack in the history of the United States - worse than Pearl Harbor.... We weren't safe," Trump said.

George W. Bush offered some vivid imagery of what took place on Sept. 11 without addressing Trump's criticism and saluted U.S. military personnel, a key constituency in South Carolina.

In his first public campaign appearance of the year for his brother, George W. Bush also met privately with South Carolina's Republican governor, Nikki Haley, who has yet to endorse a candidate for the primary vote.

George W. Bush's standing has risen among all Americans since he left power in 2009 and he has stayed on the sidelines of his brother's presidential bid, headlining private fundraisers but otherwise staying off the campaign trail.

That he is getting out in public now shows the urgency Jeb Bush sees in a good performance in South Carolina. Bush finished in sixth place in the Iowa caucuses and in fourth place in the New Hampshire primary - the first contests in the state-by-state battles to pick a party nominee for the Nov. 8 presidential election.

Jeb Bush predicted a good showing on Saturday, telling the crowd that "Saturday is going to be a surprise."

Trump is also keeping an eye on Cruz and trying to prevent him from gaining ground on him. He issued a statement accusing Cruz of dirty politics for running a negative ad against him.

Trump also threatened to sue Cruz to determine whether he can legally serve as president since he was born in Canada and therefore might not meet the requirements set out in the U.S. Constitution.

"One of the ways I can fight back is to bring a lawsuit against him," Trump said.

(Additional reporting Andrea Shalal in Washington; Editing by Frances Kerry and Dan Grebler)