(CNN) Donald Trump was supposed to be a blip in the 2016 race, at most a brief summer fling for voters looking for entertainment.

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.

Trump is joined by his family as he is sworn in as President on January 20.

Trump is joined by his family as he is sworn in as President on January 20.

Trump walks on stage with his family after he was declared the election winner on November 9. "Ours was not a campaign, but rather, an incredible and great movement," he told his supporters in New York.

Trump walks on stage with his family after he was declared the election winner on November 9. "Ours was not a campaign, but rather, an incredible and great movement," he told his supporters in New York.

Trump apologizes in a video, posted to his Twitter account in October, for vulgar and sexually aggressive remarks he made a decade ago regarding women. "I said it, I was wrong and I apologize," Trump said, referring to lewd comments he made during a previously unaired taping of "Access Hollywood." Multiple Republican leaders rescinded their endorsements of Trump after the footage was released.

Trump faces Democratic nominee Hillary Clinton in the first presidential debate, which took place in Hempstead, New York, in September.

Trump delivers a speech at the Republican National Convention in July, accepting the party's nomination for President. "I have had a truly great life in business," he said. "But now, my sole and exclusive mission is to go to work for our country -- to go to work for you. It's time to deliver a victory for the American people."

Trump delivers a speech at the Republican National Convention in July, accepting the party's nomination for President. "I have had a truly great life in business," he said. "But now, my sole and exclusive mission is to go to work for our country -- to go to work for you. It's time to deliver a victory for the American people."

Trump speaks during a campaign event in Evansville, Indiana, on April 28. After Trump won the Indiana primary, his last two competitors dropped out of the GOP race.

Trump speaks during a campaign event in Evansville, Indiana, on April 28. After Trump won the Indiana primary, his last two competitors dropped out of the GOP race.

The Trump family poses for a photo in New York in April.

The Trump family poses for a photo in New York in April.

Trump -- flanked by U.S. Sens. Marco Rubio, left, and Ted Cruz -- speaks during a CNN debate in Miami on March 10. Trump dominated the GOP primaries and emerged as the presumptive nominee in May.

Trump -- flanked by U.S. Sens. Marco Rubio, left, and Ted Cruz -- speaks during a CNN debate in Miami on March 10. Trump dominated the GOP primaries and emerged as the presumptive nominee in May.

Trump appears on stage with singer Nick Jonas and television personality Giuliana Rancic during the 2013 Miss USA pageant.

Trump appears on stage with singer Nick Jonas and television personality Giuliana Rancic during the 2013 Miss USA pageant.

Trump speaks in Sarasota, Florida, after accepting the Statesman of the Year Award at the Sarasota GOP dinner in August 2012. It was shortly before the Republican National Convention in nearby Tampa.

Trump speaks in Sarasota, Florida, after accepting the Statesman of the Year Award at the Sarasota GOP dinner in August 2012. It was shortly before the Republican National Convention in nearby Tampa.

Trump poses with Miss Universe contestants in 2011. Trump had been executive producer of the Miss Universe, Miss USA and Miss Teen USA pageants since 1996.

Trump poses with Miss Universe contestants in 2011. Trump had been executive producer of the Miss Universe, Miss USA and Miss Teen USA pageants since 1996.

Trump appears on the set of "The Celebrity Apprentice" with two of his children -- Donald Jr. and Ivanka -- in 2009.

Trump appears on the set of "The Celebrity Apprentice" with two of his children -- Donald Jr. and Ivanka -- in 2009.

For "The Apprentice," Trump was honored with a star on the Hollywood Walk of Fame in January 2007.

For "The Apprentice," Trump was honored with a star on the Hollywood Walk of Fame in January 2007.

Trump wrestles with "Stone Cold" Steve Austin at WrestleMania in 2007. Trump has close ties with the WWE and its CEO, Vince McMahon.

Trump wrestles with "Stone Cold" Steve Austin at WrestleMania in 2007. Trump has close ties with the WWE and its CEO, Vince McMahon.

Trump attends the U.S. Open tennis tournament with his third wife, Melania Knauss-Trump, and their son, Barron, in 2006. Trump and Knauss married in 2005.

Trump attends the U.S. Open tennis tournament with his third wife, Melania Knauss-Trump, and their son, Barron, in 2006. Trump and Knauss married in 2005.

Trump attends a news conference in 2005 that announced the establishment of Trump University. From 2005 until it closed in 2010, Trump University had about 10,000 people sign up for a program that promised success in real estate. Three separate lawsuits -- two class-action suits filed in California and one filed by New York's attorney general -- argued that the program was mired in fraud and deception. Trump's camp rejected the suits' claims as "baseless." And Trump has charged that the New York case against him is politically motivated.

A 12-inch talking Trump doll is on display at a toy store in New York in September 2004.

A 12-inch talking Trump doll is on display at a toy store in New York in September 2004.

An advertisement for the television show "The Apprentice" hangs at Trump Tower in 2004. The show launched in January of that year. In January 2008, the show returned as "Celebrity Apprentice."

An advertisement for the television show "The Apprentice" hangs at Trump Tower in 2004. The show launched in January of that year. In January 2008, the show returned as "Celebrity Apprentice."

Trump dips his second wife, Marla Maples, after the couple married in a private ceremony in New York in December 1993. The couple divorced in 1999 and had one daughter together, Tiffany.

Trump dips his second wife, Marla Maples, after the couple married in a private ceremony in New York in December 1993. The couple divorced in 1999 and had one daughter together, Tiffany.

Trump and singer Michael Jackson pose for a photo before traveling to visit Ryan White, a young child with AIDS, in 1990.

Trump and singer Michael Jackson pose for a photo before traveling to visit Ryan White, a young child with AIDS, in 1990.

Trump signs his second book, "Trump: Surviving at the Top," in 1990. Trump has published at least 16 other books, including "The Art of the Deal" and "The America We Deserve."

Trump attends the opening of his new Atlantic City casino, the Taj Mahal, in 1989.

Trump attends the opening of his new Atlantic City casino, the Taj Mahal, in 1989.

Trump uses his personal helicopter to get around New York in 1987.

Trump uses his personal helicopter to get around New York in 1987.

Trump was married to Ivana Zelnicek Trump from 1977 to 1990, when they divorced. They had three children together: Donald Jr., Ivanka and Eric.

Trump was married to Ivana Zelnicek Trump from 1977 to 1990, when they divorced. They had three children together: Donald Jr., Ivanka and Eric.

Trump wears a hard hat at the Trump Tower construction site in New York in 1980.

Trump wears a hard hat at the Trump Tower construction site in New York in 1980.

Trump attends an event to mark the start of construction of the New York Convention Center in 1979.

Trump attends an event to mark the start of construction of the New York Convention Center in 1979.

Trump stands with Alfred Eisenpreis, New York's economic development administrator, in 1976 while they look at a sketch of a new 1,400-room renovation project of the Commodore Hotel. After graduating college in 1968, Trump worked with his father on developments in Queens and Brooklyn before purchasing or building multiple properties in New York and Atlantic City, New Jersey. Those properties included Trump Tower in New York and Trump Plaza and multiple casinos in Atlantic City.

Trump stands with Alfred Eisenpreis, New York's economic development administrator, in 1976 while they look at a sketch of a new 1,400-room renovation project of the Commodore Hotel. After graduating college in 1968, Trump worked with his father on developments in Queens and Brooklyn before purchasing or building multiple properties in New York and Atlantic City, New Jersey. Those properties included Trump Tower in New York and Trump Plaza and multiple casinos in Atlantic City.

Trump, center, wears a baseball uniform at the New York Military Academy in 1964. After he graduated from the boarding school, he went to college. He started at Fordham University before transferring and later graduating from the Wharton School, the University of Pennsylvania's business school.

Trump, center, wears a baseball uniform at the New York Military Academy in 1964. After he graduated from the boarding school, he went to college. He started at Fordham University before transferring and later graduating from the Wharton School, the University of Pennsylvania's business school.

Trump, center, stands at attention during his senior year at the New York Military Academy in 1964.

Trump, center, stands at attention during his senior year at the New York Military Academy in 1964.

Trump, left, in a family photo. He was the second-youngest of five children.

Trump, left, in a family photo. He was the second-youngest of five children.

Trump at age 4. He was born in 1946 to Fred and Mary Trump in New York City. His father was a real estate developer.

Trump at age 4. He was born in 1946 to Fred and Mary Trump in New York City. His father was a real estate developer.

President-elect Donald Trump has been in the spotlight for years. From developing real estate and producing and starring in TV shows, he became a celebrity long before winning the White House.

President-elect Donald Trump has been in the spotlight for years. From developing real estate and producing and starring in TV shows, he became a celebrity long before winning the White House.

"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.

Photos: Donald Trump: His own words Photos: Donald Trump: His own words Hide Caption 1 of 6 Photos: Donald Trump: His own words Hide Caption 2 of 6 Photos: Donald Trump: His own words Hide Caption 3 of 6 Photos: Donald Trump: His own words Hide Caption 4 of 6 Photos: Donald Trump: His own words Hide Caption 5 of 6 Photos: Donald Trump: His own words In his interview with CNN's Dana Bash, Donald Trump lashes out at a lawyer who had requested a break to pump breast milk in the middle of a deposition. Hide Caption 6 of 6

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.

Photos: The week in politics Photos: The week in politics Former U.S. Secretary of State Hillary Clinton testifies before the House Benghazi Committee on Thursday, October 22. Clinton mounted a passionate defense of her response to the attack, which claimed the lives of four Americans in 2012. But she came under repeated criticism from Republicans who tried to prove she ignored pleas from U.S. diplomats for better security. Hide Caption 1 of 16 Photos: The week in politics U.S. Rep. Paul Ryan walks to his office after a news conference on Capitol Hill on Tuesday, October 20. The Wisconsin Republican is pushing ahead with his bid to be the next House speaker. Hide Caption 2 of 16 Photos: The week in politics A U.S. Marine stands at attention as Air Force One prepares to depart Andrews Air Force Base in Maryland on Wednesday, October 21. Hide Caption 3 of 16 Photos: The week in politics Supporters of Republican presidential candidate Ben Carson stand in front his tour bus at a book signing in Edmond, Oklahoma, on Wednesday, October 21. Hide Caption 4 of 16 Photos: The week in politics U.S. Vice President Joe Biden turns to his wife, Jill, after announcing Wednesday, October 21, that he would not be running for President. The announcement took place at the White House Rose Garden with President Barack Obama. Hide Caption 5 of 16 Photos: The week in politics Hillary Clinton is reflected on a teleprompter during a campaign rally in Alexandria, Virginia, on Friday, October 23. Hide Caption 6 of 16 Photos: The week in politics Former U.S. Sen. Jim Webb waits in a side room Tuesday, October 20, before announcing that he would no longer seek the Democratic Party's nomination for President. He told reporters in Washington that he would consider an independent bid. Hide Caption 7 of 16 Photos: The week in politics Republican presidential candidate Donald Trump speaks during a campaign rally in Burlington, Iowa, on Wednesday, October 21. Hide Caption 8 of 16 Photos: The week in politics U.S. Sen. Bernie Sanders, who is seeking the Democratic Party's nomination for President, appears on Jimmy Kimmel's late-night show in Brooklyn, New York, on Wednesday, October 21. Hide Caption 9 of 16 Photos: The week in politics Democratic presidential candidate Martin O'Malley rolls up his sleeves Wednesday, October 21, as he arrives to host an event at the Microsoft Innovation & Policy Center in Washington. Hide Caption 10 of 16 Photos: The week in politics President Barack Obama holds a pen as he vetoes a defense-spending bill on Thursday, October 22. Obama said the $612 billion National Defense Authorization Act used monetary tricks to circumvent mandatory spending caps that both Democrats and Republicans want to see eliminated. Hide Caption 11 of 16 Photos: The week in politics Republican presidential candidate Jeb Bush speaks at a forum in North Las Vegas, Nevada, on Wednesday, October 21. Hide Caption 12 of 16 Photos: The week in politics U.S. Secretary of State John Kerry poses with a guitar that was given to him by Spanish Foreign Minister Jose Manuel Garcia-Margallo during a meeting in Madrid on Sunday, October 18. Hide Caption 13 of 16 Photos: The week in politics Former Rhode Island Gov. Lincoln Chafee ends his bid for President while speaking at the Democratic Party's annual Women's Leadership Forum on Friday, October 23. Hide Caption 14 of 16 Photos: The week in politics Republican presidential candidate Rick Santorum poses with a veteran during a forum in Plano, Texas, on Sunday, October 18. Hide Caption 15 of 16 Photos: The week in politics President Obama shares a laugh with Agatha Sofia Alvarez-Bareiro, a high-school senior from New York, as he prepares to look at the moon during White House Astronomy Night on Monday, October 19. The event brought together students, teachers, astronomers, engineers, scientists and space enthusiasts for an evening of stargazing. Hide Caption 16 of 16

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."