Spartanburg, South Carolina (CNN) Jeb Bush on Tuesday defended a 2005 letter that he wrote praising the work of Swift Boat Veterans for Truth, a group that questioned then-Sen. John Kerry's military service record in Vietnam.

Speaking at an event in Spartanburg, South Carolina, Bush also argued that if Republicans accept Donald Trump's "ugly" and "mean-spirited" language, then "we'll never win."

The former Florida governor was asked by a panel, which was hosted by the Palladian View in Spartanburg, about the Swift Boat attacks against Kerry in light of the recent attack on John McCain by Trump -- who, like Bush -- is running for president.

Bush confirmed he sent the letter to the head of the group, Col. George "Bud" Day, a highly decorated veteran who was McCain's cellmate in Vietnam, and thanked him for his service. Bush added that he's not backing down from what he wrote.

"He won every award possible," Bush said, talking about Day's military record. "He served in three wars and if he says that there was a problem, I believed him. He's a great Floridian and a great American and so I wrote him a note thanking him for his service. Not going to change my beliefs about that at all. "

"I wish Bud Day was here with us," he said.

The letter came to light after Bush criticized Donald Trump for questioning whether McCain, who was a prisoner of war for five years, should be considered a war hero. Trump has since tried to clarify, saying that McCain is a hero, but his initial comments sparked quick backlash from other Republicans.

"(McCain's) a hero, for crying out loud," Bush said Tuesday in his latest comments denouncing Trump's remarks. "I mean, it's not that complicated."

But Bush took some heat himself, with critics saying he was using a double standard by praising Swift Boat Veterans for Truth but tweaking Trump for his criticism of a veteran.

In the letter, Bush thanked Day for his "unwavering support" and thanked the group for "their willingness to stand up against John Kerry," the Democratic nominee who challenged Bush's brother, George W. Bush, in the 2004 presidential race.

Before Bush's comments Tuesday, a spokesperson for his campaign on Monday, said the "thank you letter" was "not in any way analogous to condemning Donald Trump's slanderous attack on John McCain."

Also at the forum on Tuesday, Bush was asked by the panel about his thoughts on Trump and how the real estate titan and reality star was affecting the 2016 race. The event came just hours after Trump held an event in Hilton Head, South Carolina, where he blasted the state's senior senator, Lindsey Graham, as an "idiot" and a "light weight."

Bush said he understands why Trump is able to tap into the anger felt by voters about issues like immigration and sanctuary cities, but he argued that Trump's language was damaging to the Republican Party.

"The problem with Mr. Trump's language is its divisive, it's ugly, it's mean-spirited," he said.

"We have to separate him from the people that have legitimate concerns about the country, and I think if you do that, then his campaign - -and creating an active kind of group of people (who are) very concerned -- that maybe more active is a good thing," he said. "But if we embrace this language of divisiveness and ugliness we'll never win."

Bush also mentioned the latest candidate to join the GOP race, Ohio Gov. John Kasich, who announced his presidential bid earlier Tuesday and is expected to compete for the same base of voters.

Talking about rules that will limit the first Republican debate next month to 10 candidates, Bush argued that it's a "strange situation" that Kasich, who's polling in the low single digits, may find himself left out.

"I think it's odd," he said, praising Kasich as an "effective governor" who has a "great" record. "But I'm looking forward to being a participant in it. And I'll give a shout out to Kasich if he's not on. Maybe he'll be in the crowd if he's not in there."

Bush joked about the large number of Republican candidates in race, saying he's been unsuccessful in scaring away other people from running, as some predicted he might with his high name recognition and vast network.

"That proved that wrong didn't it?" he said with sarcasm, as the audience laughed. "I'm really intimidating in the field."

Asked how he'll combat concerns about a third Bush presidency, the former governor reiterated he's aware it will be an issue for some people and he will campaign hard to "show his heart."

But he added that he's someone with a real record as governor of a purple state and who's developed a thick resume in business as well.

"It's not like I've been kind of in some test tube waiting for my chance now -- the third Bush. It's not like that," he said. "I've actually had a life."