As Democrat Kamala Harris retained a small lead in some opinion polls in September, Republican Donald Trump leaned into dark rhetoric about migrants. Some donors and advisors said at the time they feared the tactic would destroy his campaign by alienating independent voters.

Yet the rhetoric kept the spotlight on immigration, an issue that favoured Trump more than Harris, according to opinion polls.

At his September 10 debate with Harris, their only face-to-face showdown, Trump repeated false claims that Venezuelan gangs had taken over swathes of a Colorado town. And he championed a false rumour that Haitians in Ohio were stealing and devouring their neighbours’ pets. “They’re eating the dogs!” Trump shouted. “They’re eating the cats!”

As those lines went viral, donors urged the campaign to focus on other issues.

After the debate, the screens at his rallies featured big slides flashing what appeared to be computer-generated images of Venezuelans in Colorado apartment buildings. “Migrant crime” became a hallmark of Trump’s campaign, though academic studies show immigrants do not commit crimes at a higher rate than native-born Americans. Trump frequently put the spotlight on young white women allegedly killed by migrants illegally in the country.

The extreme rhetoric, though divisive, diverted attention from issues where Trump was vulnerable, such as abortion or the January 6, 2021, attack on the US Capitol. While immigration has long been a hot-button social issue in the US, Trump spun it as an urgent existential threat, based largely on unfounded conspiracy theories.

As voters responded – some with support, others with sharp criticism – Trump succeeded in injecting immigration deeper into the race.

As his polling numbers started to improve in October, some advisors and donors praised Trump: They now believed he had shifted the focus in his favour.

“I do think as clumsy and as weird as the animal-eating conversation has been, it did put a big magnifying glass on what is happening in some communities in the country that are being overwhelmed,” said Roe, the Republican strategist.

One advisor put it more bluntly. On immigration, he quipped, Trump was “as dumb as a fox,” referring to the animal known for its cunning.

As Trump’s polling numbers stabilized in October, supporters worked in the background to broker alliances that helped him consolidate support in the battleground states, especially among young men who don’t regularly vote.

Trump donor and financier Omeed Malik helped secure former independent presidential candidate Robert F. Kennedy Jr’s endorsement after Kennedy dropped out of the race in August. Malik, a former Democrat, is part of a group of rich tech investors who have shifted right and embraced Trump in recent years. The Kennedy endorsement, Malik says, “helped build an alliance we haven’t seen in modern American politics.”

But no single figure did as much to boost Trump’s campaign as Elon Musk, CEO of Tesla and SpaceX and the world’s richest man. Over the summer, a Musk-founded super PAC, an outside spending group that can raise unlimited sums, emerged to help Trump turn out voters.

As Harris held on to a small lead in some polls, some Trump donors and Republican political operatives had raised doubts not just about Trump’s rhetoric, but also about the structure of his campaign. A common complaint centred on Trump’s “ground game,” the network of volunteers and employees who go door-to-door advocating for a candidate.

Trump had opted for a lean ground game that targeted a specific universe: infrequent voters inclined to support Trump. Senior Trump campaign staff had publicly described previous Republican door-knocking operations as bloated and expensive. In past elections, they said, Republicans wasted resources visiting people whose minds were already made up.

As the race tightened, Trump allies grew concerned that a weak swing-state operation could cost them the election, they said. One prominent Trump ally, Representative Marjorie Taylor Greene, warned Trump over the summer that his ground game appeared almost non-existent in her state of Georgia, said a person close to both politicians.

A representative for Greene did not immediately respond to a request for comment.

Musk poured at least $119 million into a Trump door-knocking operation, according to filings to the Federal Election Commission.

While Musk’s America PAC struggled to reach some of its door-knocking goals and faced fraud claims, the massive operation still helped Trump’s otherwise paltry ground efforts. Like much of the campaign, Musk’s operation was geared at motivating soft Trump supporters to head to the polls – not at persuading undecided voters.

Musk’s ownership of X also helped. The billionaire regularly dashed off posts supporting Trump and spread misinformation about voting to his more than 203 million followers. Musk and Trump also spoke regularly, according to a source with knowledge of the conversations. Trump publicly promised to tap Musk to lead a government efficiency commission if elected.

By October, Musk had set up camp in must-win Pennsylvania and was giving away $1 million each day to a voter who signed a conservative political petition. Only those registered to vote in one of the seven swing states would be eligible to sign the petition.

Musk did not respond to an email requesting comment.

By the end of the campaign, Trump’s rhetoric was turning angrier and more apocalyptic. He frequently warned of a global nuclear war should he lose. In the final weeks, he appeared to gain momentum in polls. With the race looking like a coin toss, he showed little restraint. He repeatedly warned of the “enemy from within” when referring to political opponents, words that Democrats denounced as dangerous and reminiscent of fascist rhetoric.

His speeches went off on increasingly odd tangents. In Pennsylvania in late October, Trump discussed the purported size of the late golfer Arnold Palmer’s penis. In a discussion with podcaster celebrity Joe Rogan a few days later, he mused about life on Mars and said he would like to be a “whale psychiatrist” when talking about wind energy.

Trump’s aides said he set the pace of his events and talked as long as he wanted, often in an unscripted style he called “the weave” – a meandering approach that he claimed always returned to his initial point.

At a rally at Madison Square Garden in New York 9 days before Election Day, a pro-Trump comedian called Puerto Rico a “floating island of garbage” – a statement that sparked an instant backlash and risked turning off the key Puerto Rican vote in crucial battleground states. The event was supposed to showcase Trump’s broad-based coalition. Instead, opponents branded it a symbol of the bigotry voiced by some of his supporters.

Two days later, a week before Election Day, Trump got a break – and ran with it. Responding to the comedian’s insult, Biden seemingly referred to Trump supporters as “garbage.” Trump’s showman instincts kicked in. Looking to draw attention to the gaffe, he donned an orange safety vest and climbed into a garbage truck before a sea of cameras in Wisconsin. The moment went viral on social media, possibly distracting some voters from his vulnerable issues.

Beyond these erratic flourishes in the final days, however, Trump regularly made a point of asking supporters whether they were better off during his presidency or the current Biden-Harris administration.

In the end, a critical mass of American voters fell into one of two camps. They either positively embraced Trump’s dark vision, or they were willing to overlook it.

“If you were going to be turned off by Trump’s rhetoric, you’d already be a Harris supporter,” said Republican consultant Jon Fleischman. It was the economy, Fleischman argued, that in the final hours led undecided voters to break for Trump.

“Voters looked back and asked the question: Am I better off now than I was four years ago? And I think most swing voters said: ‘No.’”

• A Reuters report