But that summer fling has decidedly turned into the autumn of Trump, and Republican Party leaders increasingly acknowledge that Trump lingering in first place into the winter is no longer a laughable possibility. As the GOP candidates prepare for their next debate Wednesday, the only check on Trump's dominance has come in Iowa from Ben Carson -- a fellow outsider.
Trump's months-long dominance has come at the expense of establishment favorites like Jeb Bush and Marco Rubio, who are struggling to shine next to the billionaire's larger-than-life personality and the media's insatiable appetite for all things Trump.
"Professional Republicans keep holding their breath that the race will dramatically shift but many of them are beginning to turn blue in the face from waiting," said South Carolina GOP Chairman Matt Moore. "And so it's a very confusing time right now."
Bush, the heir to a political dynasty widely expected to stay near the top of the race, has perhaps suffered the most. With his poll numbers dragging, his campaign announced drastic cost-cutting measures
last week that only fueled Trump's insults.
"Here is a guy who wants to run our country and he can't even run his own campaign," Trump said in Florida this weekend, referring to Bush.
The New York real estate mogul has managed to rise to the top -- and stay there -- all while rejecting the norms of presidential campaigning.
Throughout the race, Trump has reeled off flamboyant comments that might have spelled death for another candidate. Recently, he crossed a seemingly uncrossable line when he suggested President George W. Bush may have been to blame for the September 11, 2001, terrorist attacks that brought down the World Trade Center.
And little more than three months before the Iowa caucuses, Trump has still not run a single TV ad, held fundraisers and solicited checks from donors, or shown deference to local powerbrokers in early states. His supporters, meanwhile, seem impervious to — and at times egged on by — the candidate's penchant for inflammatory remarks.
Trump has taunted Arizona Sen. John McCain for being captured in the Vietnam War, made off-color comments about Fox News host Megyn Kelly that were widely viewed as sexist, and joked about rival Carly Fiorina's appearance. Earlier this month, Trump seemed to ridicule Carson for his religious affiliation (Carson is a Seventh Day Adventist.)
The candidate's momentum despite all of this has party leaders wondering what, if anything, will slow Trump down.
"I don't think you can find a single Republican in this state that could have predicted that four months ago, Donald Trump would still be in first place (nationally)," said Iowa GOP Chair Jeff Kaufmann, who attributed Trump's success to a "hunger for an outsider."
The only sign that Trump's momentum may be slowing has come from Iowa, where another candidate with an unorthodox campaign, neurosurgeon Carson, has inched ahead of him in two recent polls. But Carson may be even less connected to the political establishment than Trump, and Trump remains ahead both nationally and in other early states like New Hampshire and South Carolina.
Rubio, speaking to CNN's Jamie Gangel in an interview that aired Sunday on "State of the Union," is confident Trump's popularity will wane.
"I'm going to support the Republican nominee, and I'm comfortable that it's not going to be Donald Trump, and I'm increasingly confident that it's going to be me," he said.
Until recently, many of Trump's opponents appeared hesitant to directly engage or hit back at the candidate (and if a few lower-tier candidates like Lindsey Graham and George Pataki have torn into Trump, those attacks have done nothing to lift their own political fortunes).
9/11 comments rile Republicans
The billionaire's remarks about Bush's role in 9/11 marked one of Trump's most striking departures yet from traditional GOP thinking. Since 2001, Republicans have defended Bush's handling of 9/11 with pride. Even after he left office, as conservatives have critiqued Bush's legacy on government spending and immigration, his record on terrorism has been largely off- limits.
The comments were also a sharp jab at Trump's rival, Jeb Bush. Since becoming a White House candidate, Bush has fiercely defended his brother's presidency and legacy, which Trump has increasingly delighted in making a political target.
But if the latest Trump controversy has riled up Republicans, there were few signs that his supporters are bothered. At campaign events in South Carolina and Iowa last week, Trump fans came to the candidate's defense and slammed the media for misconstruing the intent of his comments.
"He just simply meant that if it were on his watch, he wouldn't have let it happen because he would have secured the borders," said Kermit Forth, a 65-year-old correctional officer from Crawfordsville, Iowa, who attended Trump's campaign rally in Burlington last Wednesday.
Some party leaders have given Trump a wide berth to make his provocative comments. Rudy Giuliani, who was mayor of New York City during the attacks, said laying the blame for 9/11 on any one president was simply "not fair." Giuliani, who has not yet endorsed a 2016 candidate, lamented that bickering among the GOP candidates on this issue was unproductive.
"I do not blame it on any American. I blame September 11 on the people who did it and the people who organized it," Giuliani told CNN in an interview, adding about Trump's comments: "It is an unusual statement and I don't agree with it."
Still, Giuliani declined to criticize Trump more bluntly, calling the candidate a longtime personal friend. And he said Trump is doing the party a favor by bringing a "tremendous amount of attention" to the GOP primary.
"If some of the people in the party are freaking out, they're making a mistake. This is the best thing that could happen to the Republican Party," Giuliani said. "I wish when I ran for president in 2007 and 2008, I had 24 million people I could have talked to."
Others were not as kind.
"He's totally wrong. I think he's wrong as a Republican and he's wrong as an American," New York GOP Rep. Peter King told CNN regarding Trump's 9/11 comments. "To me, if this is his real thinking, it makes him unfit to be commander-in-chief."
It's still early
In public, allies of candidates like Bush and Rubio insist it's much too early in the cycle to draw any meaningful conclusions about the GOP race. In private, they say they are certain that political gravity will eventually kick in for Trump.
Supporters of Bush and Rubio, among other candidates, point out that their campaigns are laying down the groundwork to run a long-distance marathon by expanding their donor networks, building grassroots operations in the early states and picking up endorsements across the country.
Despite Trump's persistent lead, plenty of GOP elites persist in their belief that he will ultimately turn out to be a sideshow.
"It reminds me of four years ago -- I don't know who was leading the race at the time but it wasn't Romney. Eight years ago, I don't know who was leading the race at the time, but it wasn't John McCain," said Iowa Republican State Sen. Charles Schneider, who is supporting Jeb Bush. "It's still early ... and that's why it doesn't really concern me."