Businessman Donald Trump has defied the expectations of political operatives and pundits alike, turning his summer surge in the GOP presidential polls into full-fledged domination, even expanding his share of the vote in new surveys this week.

Now the billionaire, who has not had to dip deeply into his own pocket, says he’s ready to spend tens of millions of dollars to defend himself on the airwaves against an anticipated onslaught from fellow Republicans increasingly worried he may emerge as their party’s nominee next year.

Widely mocked as a political sideshow in June, Mr. Trump has now topped every major national and state poll of Republican presidential contenders since July, hitting his highest total yet in an NBC News/Wall Street Journal survey this week.

“I have been a winner. I have always won. I win even if the world goes bad. I win,” Mr. Trump told The Washington Times in a telephone interview Tuesday, explaining how he’s defied the pundits with his staying power.

Top GOP figures no longer dismiss his bid as a fluke.

“Obviously it has evolved from someone who we didn’t think was real serious to someone who clearly is,” Arizona Sen. John McCain, the 2008 GOP presidential nominee, said Tuesday.

Most striking is how Mr. Trump has handled gaffes and attacks that many operatives had figured would dent him. A July dust-up with Mr. McCain over the senator’s time in a Vietnamese prisoner-of-war camp did not hurt Mr. Trump, nor did an interview he gave to Rolling Stone where he commented on fellow candidate Carly Fiorina’s looks and electability.

Even after Mrs. Fiorina dinged him in the second GOP debate for the remarks, Mr. Trump didn’t slide. Instead, the NBC/Wall Street Journal poll showed he and Mrs. Fiorina headed in opposite directions, with Mr. Trump rising 4 percentage points, to 25 percent, while she slid 4 points to 7 percent.

“I must have won the debates,” Mr. Trump said.

As a result, some Republicans have wondered whether they should ramp up efforts to chop Mr. Trump down.

He said he is aware of the possibility, but warned that an orchestrated GOP effort to derail his candidacy would backfire with voters who won’t listen to the “lies” that are spewed by the special interests and lobbyists that are rallying to his rivals.

“I don’t think the public will stand for it,” he said. “We are democracy, and I don’t think something like that will pass muster.”

Mr. Trump also said that he had anticipated spending upwards of $25 million up until this point in the race on campaigns ads but has not spent a penny. He said he will spend at least that much to counter negative attacks on him — but said he’s learned that his own attacks are better than ads.

“If someone does a negative ad, I am going to attack them very strong,” he said. “I find that an attack when I do it personally is better than a negative ad.”

Mr. Trump’s dominance in the 2016 GOP polling breaks with the trend in recent presidential elections, where voters tested various candidates, sending them rising and falling in the polls as conservatives and establishment-minded Republicans battled for primacy.