The crowd of 800 cheered as pyrotechnics lit up the theater stage in Clacton-on-Sea, a faded English seaside town, and the lyrics of an Eminem song boomed out: “Guess who’s back, back again?”

The answer is Nigel Farage, supporter of former President Donald J. Trump, Brexit campaigner and Britain’s best known political disrupter, who leads a new insurgent party that rails against immigration and promises to upend Britain’s impending general election. Never before elected to the British Parliament, Mr. Farage is running to represent the Clacton area, and opinion surveys suggest he has a strong chance of winning.

“The establishment are terrified, the Conservatives are terrified,” Mr. Farage declared gleefully, referring to the governing party, which trails badly in opinion polls ahead of the July 4 election. Britain was “a broken nation,” he added, in a speech attacking targets ranging from asylum seekers to the BBC.

A polarizing, pugilistic figure and a highly skilled communicator, Mr. Farage, 60, helped the Conservatives to a landslide victory in the last general election by not running candidates from his Brexit Party in certain key areas.

This time his plan is rather different. He wants to destroy the Tories by poaching much of their vote, then replace — or take over — the party’s remnants. Early in the campaign, after a journalist asked if he wanted to merge Reform with the Conservatives, he replied: “More like a takeover, dear boy.”

Mr. Farage has tried and failed seven times to be elected to Britain’s Parliament, and his party, Reform U.K., is unlikely to win more than a handful of seats under an electoral system that punishes small parties.

But for two decades he has shaped Britain’s political conversation, driving the Brexit cause, outflanking the Tories and pushing them further right. A win in Clacton could anoint him as a power broker in the battle for the soul of the Conservative Party.

In an interview last week in his Clacton campaign office above an arcade, Mr. Farage was relaxed and confident, joking and clearly enjoying his return to frontline politics.

“The thing we look to is Canada in ’93 where Reform in effect reverse took over the existing conservative party,” said Mr. Farage, referring to 1993 elections when another insurgent party, after which Reform U.K. is named, helped crush the established Progressive Conservatives. “That’s a possible model,” he said, adding that, alternatively, Reform might grow organically over the next five years.

‘The end of the line’

Clacton is the last stop on a rail line northeast of London. Its history has been intertwined with Mr. Farage’s since 2014, when a Conservative lawmaker, Douglas Carswell, defected to the populist U.K. Independence Party, which Mr. Farage then led. Mr. Carswell won re-election, becoming one of two lawmakers ever to represent UKIP in Britain’s Parliament.

The town, which had high levels of unemployment and poverty, became a focus for policymakers and columnists trying to understand the surge of populist politics. In 2016, Clacton voted by almost 70 percent to leave the European Union.

Mr. Farage told me Clacton was “the end of the line” but also a place where folk “feel very, very, English, really identify with being English — and there is obviously some mourning for the glory days of the seaside.”

In 2019 Giles Watling, a Conservative, was elected with 72 percent of the vote. In normal times, his 25,000 majority would be almost unassailable. But these are not normal times for the Conservatives. And in the 2019 election, Mr. Farage’s Brexit Party did not run here.

Mr. Watling did not respond to requests for an interview but Chris Griffiths, a Conservative activist, conceded that Mr. Farage “had lit up what was probably going to be a very drab campaign.”

Maurice Alexander, another Conservative activist, whose parents came to Britain from Belgium around the time of the Second World War, was less positive about Mr. Farage. “He frightens me,” he said.

Immigration featured prominently in Mr. Farage’s speech in Clacton, as in most of his speeches. “Our quality of life has diminished for every single one of us as a result of this population explosion,” he thundered, adding, “The time has come to stand up and say ‘enough is enough.’”

‘The consequences of what he says have outcomes’

Earlier that day, the Labour Party candidate, Jovan Owusu-Nepaul, was knocking on doors.

“There is a lot of anti-Farage, there is a lot of pro-Farage,” said Mr. Owusu-Nepaul, 27, who was born in Birmingham and has Ghanaian and Jamaican heritage. He said the reception from voters had been largely positive, but that he was also sometimes a victim of the “dog whistle stuff that Farage has been quite effective at using.” Just that day, he had been told, “Go home, we don’t want you here, immigrants out,” he recalled.

“I think Farage coming here has inflamed a level of tension,” he added. “The consequences of what he says have outcomes — and they have outcomes for people like me, they have outcomes for people like my friends, they have outcomes for people I don’t even know exist in this country, but will be kind of tormented by the type of rhetoric he espouses.”

Mr. Farage has criticized a 2019 social media conversation in which an American student posted: “Going into 2020 I’m going to continue to be vocal about how to tackle racism and the fact I drink white man tears on a regular basis.” Mr. Owusu-Nepaul replied to the comment with: “My favourite drink.”

Mr. Owusu-Nepaul said his comment was a joke, taken out of context.

On the morning of his rally, Mr. Farage visited a breakfast club for veterans in Clacton where he enjoyed a fry-up of bacon, fried bread, black pudding and baked beans. “He is listening to veterans, he knows what we want,” said David Bye, who served in the navy and organizes the club.

Other fans include Lynn Tuckwell, a retired beautician who attended a later meeting. A Brexit voter, she said she felt disappointed with the results but did not blame Mr. Farage. “It wasn’t Nigel’s fault, the Conservatives didn’t deliver Brexit. Nigel got us Brexit, and he’s come out of retirement to get us out of this mess.”

Nationally, Mr. Farage prompts distaste as well as admiration. After saying that the expansion of the European Union and NATO had provoked the war in Ukraine, he was fiercely criticized across the political spectrum.

Reform’s candidates have crossed other lines, with one saying that Britain should have remained neutral in the fight against the Nazis, and another using antisemitic tropes and claiming that Jewish groups were “agitating for the mass import into England of Muslims.”

What kind of lawmaker Mr. Farage might be is unclear. In a February interview with the London Times, when he was still weighing whether to run for Parliament, he mused: “Do I want to spend every Friday for the next five years in Clacton?”

Mr. Farage said that was not a slur on the town but a rhetorical question about returning to frontline politics. “I was saying that’s the decision — a very big decision — and I have decided ‘Yes.’”

Mr. Carswell, now chief executive officer of the Mississippi Center for Public Policy, thinks Mr. Farage will probably win in Clacton but urged him to focus on granular campaigning.

“You don’t need to have a gathering of 1,000 people in a public hall,” said Mr. Carswell, speaking by phone from the United States, adding: “You need to persuade the people who didn’t come to your launch rally, persuade the people who don’t follow you on Facebook and social media.”

Some two miles from Clacton-on-Sea is the village of Jaywick, once one of the most deprived parts of Britain and part of the area Mr. Farage is seeking to represent. Terry Haggis, 66, who lives there, remembers better days from vacations in his youth. “I feel let down, this is a holiday town, when I was a youngster it was bustling. There has not been enough money pumped in.”

Undecided on how to vote, he is leaning Conservative because he fears voting for Reform might advantage Labour. His other worry is that Clacton could help Mr. Farage more than the other way around.

“My question is: Is he going to use it to further his political career?” he said. “Is it a backdoor way to get in and do what he wants to do?”