Washington, United States--The extraordinary Donald-Trump-for-president roadshow careened into America's deep South, as the abrasive tycoon railed once more against the political establishment but offered little policy substance in a rambling speech at an Alabama rally.

The billionaire real estate mogul has snatched the lion's share of the attention and support of Republican voters, using a combative tone to lash out at other candidates with a coarseness rarely seen at the top tier of American campaigns.

And he has left his rivals flummoxed over how to contain the political brute now turning the presidential race on its head.

With more than five months before Iowa and New Hampshire cast the early votes in the party nominating process, Trump leads his 16 Republican rivals in polls across the board.

He is a wildly hyperbolic, hugely entertaining non-politician -- and his anti-establishment clarion call has quickly become the loudest voice in US politics.

It rang out in jarring fashion at a football stadium in Alabama, where several thousand fans gathered for Trump's latest iteration of a stump speech.

Patriotically donning a red baseball cap, white shirt and blue blazer, Trump lashed out at America's existing immigration system, in particular its birthright citizenship provision that allows anyone born in the United States to be a citizen, even if their parents are undocumented immigrants.

"We're the only place, just about, that's stupid enough to do it," he boomed.

Trump also let loose on his main fellow Republican rivals Jeb Bush and Marco Rubio, mocking them for trailing him in a recent poll conducted in their native Florida.

"We have a governor, and we have a sitting senator, and I'm killing them," Trump said, to raucous applause.

And, while short on policy specifics, he insisted his reputation as a Grade A business titan will bring Asian powers to their knees in trade negotiations.

"I do know what I'm doing, and I don't say that in a braggadocious way," Trump said.

"The reason people like what I'm saying is because I want to put that energy, whatever the hell kind of energy it is -- I don't know if it's screwed up, if it's good, if it's ingenious, if it's what -- whatever it is, I know how to do things."

Southern states are not used to such high-profile visits by presidential candidates so early in the race, and while the stadium in mobile was not filled with the 35,000 to 40,000 supporters Trump had predicted, it was a sea of attendees soaking up The Donald.

- Vegas 'schmaltz' - 

Trump reiterated his criticism that President Barack Obama has failed to lift the "spirit" of the nation or create sufficient jobs.

"I am going to be the greatest jobs president that God ever created," Trump pledged. "You are going to be so proud."

He boasted that his lead was so overwhelming that he wanted to call snap elections.

"Can we do that? I'd like to have the election tomorrow. I don't want to wait," he said.

Trump's extraordinary early success has Republican candidates, donors and party leaders mystified.

Respected GOP strategist gave Trump's performance a thumbs down, however, describing the speech as a combination of Las Vegas schmaltz "and your Uncle Whitely making a drunk toast at a wedding."

Perhaps recognizing a need to counter the Trump machine, Bush has upped his rhetoric, snapping that Trump has "been a Democrat longer than being a Republican." 

He also cited the magnate's earlier support for abortion rights, a single-payer health care system, and a massive one-time tax on the wealthy.

The message came through in Mobile, as a plane flew over the stadium shortly before Trump's speech, pulling a banner that read: "Trump 4 higher taxes. Jeb 4 prez!"

Other candidates have gone after Trump head on, with little to show for it.

Senator Rand Paul berated him in an August 6 debate but it did minimal damage, and the Kentucky insurgent has slipped in the polls.

Paul has also released an ad attacking The Donald, but that, too, has not moved the needle.

He continues to throw elbows even as he grows in stature, despite pundits and analysts forecasting that he would be merely a passing political fad.

"It's like a science fiction film, where you shoot at him and he gets bigger," Newt Gingrich, the former US House speaker who ran for president in 2012 and, like Trump, was an early frontrunner, told Fox News this week.

"You're dealing with somebody who is totally different from anybody in modern politics."