It's still early days in the race for the Republican presidential nomination, but one thing stands out: Donald Trump is owning the microphone.

The billionaire businessman has been a veritable gusher of non-PC bombast — most notably on the topic of Mexican immigrants — since announcing his candidacy in mid-June.

On Sunday, however, Trump turned his ire on a fellow Republican, Arizona Senator John McCain, calling the decorated Vietnam veteran a "loser" and questioning whether someone who spent more than five years as a POW can really be considered a war hero.

The statement seemed to shock Americans of all political stripes, and is yet more evidence that Trump could hurt the party's chances in the next election, says Ford O'Connell, a Republican strategist.

Many Republicans are concerned that Trump "could be potentially damaging for the Republican race" and is providing ammunition for the presumed Democratic candidate Hillary Clinton as the contest becomes more focused, says O'Connell.

Trump's lack of tact and decorum shows he's really just out for himself, adds Gil Troy, a politics professor at McGill University and an expert on U.S. presidential campaigns. 

"I don't think he cares about the Republican Party — he belongs to the Party of Trump. I think he lives in the Republic of Trump," says Troy, who is also the author of the upcoming book The Age of Clinton: America in the 1990s.

'Fired up the crazies'

Trump's broadside against the senior Arizona senator was partially in response to comments McCain made in a recent interview with the New Yorker magazine, in which he said that Trump's campaign has "fired up the crazies."

Trump then fired back not only because he felt insulted, but because McCain is considered a relative moderate within the GOP, says O'Connell, who worked as a campaign adviser on McCain's 2008 run at the presidency.

GOP Trump 2016

Republican presidential candidate, real estate mogul Donald Trump speaks at the Family Leadership Summit in Ames, Iowa, on Saturday. Many of his fellow Republicans are feeling very familial toward him. (Nati Harnik / Associated Press)

O'Connell says tweaking McCain is a common tactic for any U.S. right-winger looking to score points with conservatives.

"If you want to stoke emotion with the base of the Republican Party to gain favour, John McCain's an easy target," he says. "But when you pick on his war record, that's where you cross the line."

For his part, Troy believes Trump's dismissal of McCain's military service was actually a gaffe.

"I don't think [Trump] woke up that morning saying, 'How can I disparage an American hero? How can I look like a bigger putz than I already do?'"

Troy believes that the gibe just "came out, and because he's The Donald, he doesn't retreat. Eventually, he decided, 'I will gain more by holding my position than by retreating.'"

No apologies

So far, Trump's unapologetic outbursts appear to be winning over many would-be voters, says Theda Skocpol, a political professor at Harvard and the author of The Tea Party and the Remaking of Republican Conservatism.

"He's speaking to the just-say-No portion of the Republican base," says Skocpol. "They love his anger, his anti-establishment stance, and I don't think this John McCain thing is going to diminish that."

Indeed, recent polls have put Trump in either first or second place behind Jeb Bush in the Republican field, which currently includes 15 candidates.

While the party won't formally choose its presidential nominee until July 2016, the current polls count because they can determine who participates in the Republican debates, the first of which takes place on Aug. 6 in Cleveland, and will be hosted by Fox News.

To determine the participants, Fox News will choose the 10 candidates with the highest standing in the five most recent polls. And, as O'Connell says: "If you can't get on that debate stage, you can't win the nomination."

Trump's rising approval rating could have a palpable impact on more serious-minded rivals, says Lewis L. Gould, author of Grand Old Party: A History of the Republicans.

"With Fox News debate participation depending on the polls, every point that Trump gets is coming out of the hide of one of these lesser candidates."

Playing coy

Many commentators believe Trump's criticism of McCain will ultimately harm his chances for the nomination, but Skocpol doesn't buy it.

She does believe, however, that it is galvanizing the Republican field.

Trump's remarks about McCain "offer other Republicans the opportunity to say negative things about Trump, and they weren't willing to do that when Trump was outrageously bashing Latinos and immigrants."

Indeed, fellow Republican candidates such as Jeb Bush and Marco Rubio came out with strong denunciations of Trump's statement about McCain, with Rubio telling CNN they're a "disqualifier as commander-in-chief."

Texas senator and presidential hopeful Ted Cruz, however, is one candidate who has publicly refused to engage.

"I recognize that folks in the press love to see Republican-on-Republican violence, and so you want me to say something bad about Donald Trump, or bad about John McCain or bad about anyone else," Cruz said. "I'm not going to do it."

Skocpol believes the reason Cruz is playing coy is that he "hopes to inherit Trump's support eventually."

Roller derby primaries

McGill's Troy sees Trump's outrageous candidacy as evidence of "the toxic elements of today's open-air primary derby."

"There's no sense of responsibility, there's no sense of party loyalty," he says.

Indeed, there has been growing talk that Trump could run as an independent, which many political observers say could cut into the Republican vote, the way it did for George H.W. Bush in 1992, when billionaire Ross Perot's candidacy split the conservative vote for the benefit of Bill Clinton.

But Troy says that it's foolish to try and predict Trump's next move.

In fact, Troy says he can envision a scenario where Trump loses the GOP's nomination to someone like Jeb Bush, but instead of throwing his support behind the Republican nominee, ends up backing Hillary Clinton "just because she's a more famous name and it'll get him more headlines.

"Celebrity politics are not national politics, they're not responsible politics and they're not mature politics," he says.