Bush spoke at the Dallas home of Amy and Malone Mitchell, who are members of the campaign's "Texas Leadership Committee." An audio recording of the event was provided to The Washington Post by someone in attendance, who asked for anonymity out of concern for potential retribution.
Among those in attendance Monday night was former president George W. Bush, and his wife, Laura.
"I'm typically not a very good pundit, but I'll tell you this -- what appears to be reality today will not be reality next February," the former president said as he introduced his brother to the crowd. "These things go through cycles and the winner is going to be the person who is steadiest under fire, who has had the executive experience necessary to make tough decisions. Who has got a series of plans to deal with America's most pressing problems. The winner is going to be somebody who can appeal beyond the kind narrowness of some of the politics of today. The winner is going to be somebody who's competitive. The winner is going to be somebody who's tall."
Jeb Bush's remarks generally echoed what he says publicly on the campaign trail or in interviews. He spoke broadly about his desire to "fix a few big things" and criticized the Obama administration's response to the rise of the Islamic State terror group -- as he has in recent days.
His comment on Trump came in response to a question from someone in attendance.
"I'm just curious, when are you going to make a more aggressive attack on Donald Trump?" asked an unidentified man. "It seems like in the last couple of debates -- I mean, I'm your biggest fan, but at the same time it seems like there's not a lot of fight in you. I'm just curious, come December 15, is that a possibility?"
"Come December 15, Trump will be in decline," Bush said to applause.
The next GOP presidential debate is scheduled for Dec. 15 in Las Vegas.
Recent polls of Republicans nationally and in the early contest states of Iowa, New Hampshire and South Carolina give the New York businessman a commanding lead over the GOP field -- and a double-digit lead over Bush in every instance. Since October, Bush has slipped into the single digits, while Trump has either sustained his dominance or seen it slip just slightly.
Bush's remarks on immigration came in response to a question about how he planned to win the general election. Bush told "the Jeb story," noting that he earned double-digit support from Hispanics, African Americans and other minority groups during his two elections as Florida governor. He cited his record on immigration as a good example why.
"Every other candidate, maybe with the exception of [Ohio Gov.] John Kasich is in the witness protection program, because they sense the anger and they're worried about it," he said. "So they pull back rather than persuade. I'm not going to do that. I have to be elected on my own terms. The only way you get elected is to be authentic and genuine. And I think immigration done right is a huge driver for our success ultimately as a nation. It's what makes us different in so many ways."
"A set of shared values defines our citizenship and if you enforce the laws properly and allow that to be the defining element of our society, you won't have the kinds of problems that Paris now has, where enclaves exist of people that are second-class citizens and the despair and the hatred that has built up over time occurs in those pockets that exist where people may have a French passport, may be a French citizen, but they're not really French. And America doesn't do it that way and restoring that with the proper immigration policy is something that I think is important."
"I'll take my lumps, I'm a big boy. I've got my big boy pants on," he added. "I can take the hits just as good as anybody, and I'll give it back, too."
Campaigning in Columbus, Ohio, in April, Bush told a crowd that he didn’t intend to change himself or his views: "There are views that I have that apparently are unorthodox. What am I going to do, go in a witness protection program or something?"
Bush aides declined to comment on the former governor's remarks, citing a policy of not commenting on private events.
Guest at the Monday evening fundraiser were asked to give a minimum of $500, with "Sponsors" giving at least $1,000; "Hosts" donating at least $2,700; and "Co-Chairs," or campaign bundlers, helping raise at least $20,000.
The final question of the night brought a moment of levity when a donor asked, "Whose impression do you prefer? Dana Carvey of your dad or Will Ferrell of your brother?"
"I should say Dana Carvey," Jeb Bush said, earning a big laugh.
Carvey famously lampooned former president George H.W. Bush for several years on "Saturday Night Live," while Ferrell's impressions of the younger Bush president were a program highlight for several seasons.
Jeb Bush is scheduled to campaign this week in South Carolina and New Hampshire before hosting another fundraiser in New York on Friday, according to his campaign.