First comes the throaty rumble of motorcycle exhausts as dozens of “Bikers for Trump” thunder past the sports arena on their Harley-Davidsons. Then come the chants: “Lock her up! Lock her up!” Finally, the protesters march into view, including a man in a furry pig suit, holding a placard that reads “Trump is a sexist pig”.

The sights and sounds of a Donald Trump rally are like nothing before seen or heard in a US election.

On a grey day in October, the queue stretches in four directions around the Mohegan Sun Arena in Wilkes-Barre, Pennsylvania. In more ordinary times, it hosts ice hockey games and wrestling matches. This is Trump country, on the edge of the Rust Belt, where blue-collar workers are still living in straitened times.

“Look at the people here,” says Dale Painter, a beautician wearing a cowboy hat that is decorated with the Stars and Stripes. “There are all ages and lots of women. Do you think these people were going to rallies last time?”

This is the main success of the Trump movement. For his supporters, the campaign’s existence is proof that the Republican brand can still attract new supporters. Not that many in the party leadership or among the long-standing conservative faithful view this as success. As election day approaches, they are contemplating defeat. Preparations are being made for a GOP civil war and the chance to return their movement to its founding principles.

Matt K Lewis, whose book Too Dumb to Fail charts how the party of Ronald Reagan became the party of Donald Trump, reflects on the schism between the nationalistic, anti-intellectual newcomers and the more mainstream establishment. Defeat for Trump doesn’t solve the problem, he believes. “If anyone thinks just getting rid of Donald Trump means it goes back to Paul Ryan and Marco Rubio, then they’re wrong,” Lewis says. “He’s just a symptom – or an exploiter – of problems that are deeply ingrained and pre-dated him.”

Although Trump may have built on the remnants of the grass-roots conservative Tea Party movement to secure the Republican nomination, opinion polls suggest that his strategy offers no path to the 270 electoral college votes needed to win the election.

Trump’s Republican critics argue that his protectionist, nativist agenda – characterising Syrian refugees as potential terrorists, demonising Mexicans as rapists – was failing to attract wavering voters even before a dozen women came forward to accuse him of unwanted sexual advances and worse.

The schism was laid bare when Trump’s vulgar style failed to win the backing of the Bush family and other Republican grandees. Fifty of the party’s national security experts have accused him of posing a threat to the country.

New revelations of an FBI investigation into Hillary Clinton’s private email server have given Trump some eleventh-hour impetus, but privately many Republicans are already talking of defeat. There is also discussion of creating a new conservative party.

Even Trump addressed the issue when he was asked whether the Republican Party would be beyond repair if he lost the election.

“No, I think it’s going to be a different Republican Party,” he said on Fox News on 28 October. “We have a base of people that’s so incredible – you have to see at these rallies, people get there at seven in the morning.”

Talk of the demise of the Republican Party may well be premature. It still holds a majority of governorships across the United States, as well as both houses of Congress – at least for now.

However, conservatives such as Lewis fear that the wrong lessons will be learned from Trump’s rise. Smoothing the candidate’s rough edges and quietening the misogyny while keeping the anti-immigration rhetoric and protectionist economics to placate the new supporters might yield short-term success, he says. “It could win places like Pennsylvania, even Ohio, that Trump has made a little bit more competitive than normal, without turning off college-educated Republican women.” But, he adds: “In the long run it could mean demographic death.”

The GOP already knows that. Its post-mortem report on Mitt Romney’s 2012 defeat to Barack Obama set out a blueprint for building a modern conservative movement.

“We need to campaign among Hispanic, black, Asian and gay Americans and demonstrate we care about them, too,” the report concluded. Today, this reads like Donald Trump’s game plan in reverse.

Jeanne Zaino, a professor of political science at Iona College in New York State, says that rule changes could prevent an insurgent candidate hijacking the party again. Introducing superdelegates – party officials given a vote in the nominating process – would be a way to prevent an angry base toppling mainstream candidates such as Jeb Bush or Marco Rubio. “At the end of the day, the party will do what it needs to do to protect itself,” Zaino says.

Yet not everyone is convinced. Rick Wilson, a Republican strategist and a vocal member of the “Never Trump” brigade, is already plotting a more radical, post-Republican future. Wilson is an adviser to Evan McMullin, a former CIA agent who is standing as an independent candidate in 11 states. McMullin’s more orthodox Republican positions (he favours free trade, for instance) have made him a serious challenger to Trump in Utah, a state held by the GOP since 1968.

Wilson says the breakaway group is putting in place infrastructure that could be used for a new centre-right party if the Republicans insist on following Trump – a candidate endorsed by David Duke, the former grand wizard of the Ku Klux Klan.

“If you want to be the party of Buckley and Burke,” Wilson says, referring to the 20th-century intellectual William Buckley and to Edmund Burke, considered the father of modern conservatism, “you don’t get to also be the party of [Trump’s campaign director Steve] Bannon and Duke.”