Donald Trump faced an avalanche of fresh criticism Monday for questioning Sen. John McCain's heroism. But he's getting no pressure at all from the one community that could push a candidate out of the 2016 presidential race: political donors.

The billionaire businessman is paying for his own campaign, and that means Republicans may have him around far longer than some party leaders would like.

"Nobody leaves a race because they get tired, or because they think they don't have the votes. They leave the race because they run out of money," said Frank Luntz, a GOP pollster. "Donald Trump will never run out of money, and that makes him incredibly powerful."

Indeed, Republican operatives suggest that Trump enjoys a rare freedom.

Because he doesn't need tens of millions of dollars from wealthy donors — a notoriously risk-averse crowd — the standard rules of politics simply don't apply. He can afford, literally, to continue dropping the verbal bombs that have defined his presidential campaign since the day he joined the 2016 contest in June.

At his formal announcement last month, Trump said illegal immigrants from Mexico are prisoners and rapists. Then, at a conservative summit in Iowa last weekend, he dismissed McCain's reputation as a war hero, saying of the Arizona senator who was once a prisoner in Vietnam, "I like people who weren't captured."

Critics began piling on Trump immediately, and new voices emerged on Monday, from veterans groups, Republican colleagues and President Barack Obama's spokesman, who defended McCain and called on Trump to apologize.

Paul Rieckhoff, founder of Iraq and Afghanistan Veterans of America, said Monday that Trump's "asinine comments" were "an insult to everyone who has ever worn the uniform — and to all Americans."

White House Press Secretary Josh Earnest said veterans "are entitled to an apology."

A candidate reliant on campaign contributions probably would be feeling the pain by now.

Yet the self-funded Trump has shown little sign of backing down. He leveled new criticism against the McCain on Monday, saying the senator had made America "less safe" through his votes in Congress. The real estate executive also lashed out at fellow GOP presidential aspirants who have criticized his remarks, calling them "failed politicians."

Trump said he did not need "to be lectured by any of them."

"If he were dependent on donors for his campaign, he would find the vast majority of donors would be looking for other candidates at this point," said Fred Malek, who has raised money for Republican presidential hopefuls for four decades.

Presidential competitors in both parties, and the outside groups supporting them, have already raised about $400 million for the 2016 White House contest, the majority of the money coming from donors who face no limits on their contributions.

Trump hasn't solicited a dime. Reports filed last week with federal regulators show he loaned his campaign $1.8 million. His campaign has never held a fundraising dinner where attendees pay the legal maximum of $2,700 to attend, and he has never sent email asking for others to chip in $25. Both are hallmarks of a traditional presidential candidate.

"I don't need anybody's money. It's nice," Trump proclaimed during his presidential announcement June 16 in New York City. "I'm using my own money. I'm not using the lobbyists. I'm not using donors. I don't care. I'm really rich."

Trump filed details of his personal wealth with federal regulators last week that declare a net worth in excess of $10 billion.

"Being a billionaire means you're the master of your own domain," Luntz said.

He would know: Luntz once worked for Texas businessman H. Ross Perot, perhaps the best-known presidential candidate who paid his own way. In 1992, Perot ran as an independent candidate, using almost $64 million of his fortune to get his name on ballots across the country.

This year, with Republican Party pressure building, Luntz wonders if Trump will break off and go the Perot third-party route.

"You may be able to look back on this day as the beginning" of that campaign, he said. "You make up your own rules."

On the other hand, a self-financed candidate can also be his or her own worst enemy, said Ed Rollins, Perot's onetime campaign manager.

Rollins predicted Trump would follow the same path as his former boss: off a cliff.

Perot was polling ahead of the major-party candidates in June before the election but ended up not winning a single state in November.

"He first promised to spend whatever it took to win, and we presented him with a budget of about $450 million," Rollins said. "He balked at that. He argued over every item. The Ross Perots, the Donald Trumps, they don't want to be told what to do."