Rishi Sunak’s gamble was a considerable one. Five weeks ago, the British prime minister bet the house on his belief that a summer election might offer his Conservative Party a better chance of holding onto power than waiting until the fall.

Calling a snap election served as Mr. Sunak’s last roll of the dice. But it has since emerged that in the days before he stood forlornly in the pouring rain on May 22 and told the country he was going to the polls, a number of colleagues and underlings were placing bets of the more literal kind.

Reviewing data from the week before Mr. Sunak’s announcement, bookmakers noticed a spike in bets being placed on the election date. The amounts being staked were small — totaling just a few thousand pounds — but the sudden frenzy of activity was enough to warrant further investigation.

The question of whether these bets were being made by political officials, using insider knowledge of Mr. Sunak’s intentions to make a quick profit, has come to dominate what could be the Conservatives’ final days in power. It also encapsulates how some parts of the electorate perceive the party that has governed Britain for 14 years.

“The whole thing has reinforced the public’s prior concerns,” said Luke Tryl, executive director of More in Common, a research group. “It gets right to the heart of it: ‘One rule for them, and one rule for everyone else.’”

Craig Williams, one of Mr. Sunak’s key parliamentary aides and a Conservative candidate running for office, was the first to come under scrutiny after The Guardian reported that he had placed a bet on a July election on May 19, three days before the prime minister’s announcement. Now suspended from the campaign, he has admitted to an “error of judgment,” but insisted he had not committed a criminal offense.

As the Gambling Commission, the regulator that oversees Britain’s rich and varied betting industry, extended its inquiry, a number of other senior Conservative staffers were named as being under investigation.

They included Tony Lee, the party’s director of campaigns, and his wife, Laura Saunders, a prospective Conservative candidate in the forthcoming election who has since been suspended by the party.

Nick Mason, the Conservatives’ director of data, has taken a leave of absence after being informed that he, too, is under investigation. Rumors are swirling that a number of other Conservative staffers may soon be identified by the inquiry.

One of the officers protecting Mr. Sunak, meanwhile, has been arrested over allegations that he had also made bets on the timing of the election, and the Metropolitan Police has confirmed it is investigating a number of other law enforcement officials.

The scandal is yet another blow for Mr. Sunak as he campaigns less to win the election, scheduled for July 4, than to staunch his party’s potential losses.

He had already caused an uproar after he left the 80th anniversary of D-Day commemorations early to conduct a television interview, a decision he later apologized for profusely. He then faced widespread mockery after claiming that he had known hardship as a child because his parents had not allowed him to have satellite television.

The gambling allegations have compounded that damage, polling experts said, adding to a sense of an out-of-touch party that appeared to consider itself above ethical concerns.

What was potentially most corrosive was “the perception that we operate outside the rules we set for others,” Michael Gove, one of the Conservatives’ highest profile lawmakers, told The Sunday Times. “That was damaging at the time of Partygate,” he said, referring to the scandal over lockdown-breaking parties held inside Boris Johnson’s Downing Street during the pandemic, “And it is damaging here.”

Political betting is a growing industry — more than $1.5 billion was staked on the outcome of the 2020 United States Presidential election, making it possibly the biggest single gambling event of all time — but markets on when elections might be called are, insiders say, inherently niche.

They are run, effectively, as novelties, designed to attract publicity and hopefully new customers, according to one longstanding political betting expert, who asked to remain anonymous because of the sensitivity of the industry.

They are not designed, he said, to generate vast returns. Bookmakers aim simply not to lose money on them, working on the assumption that there will be people — not just lawmakers but various party apparatchiks — who have access to better information than them. To restrict their losses, they limit the amount of money anyone can stake on the market.

The bets made in the days just before Mr. Sunak’s announcement fit that bill. Mr. Williams, for example, is accused of wagering just £100 ($125), for winnings that would have stretched to just a few hundred pounds. “They are not life-changing sums for senior figures in politics,” said Joe Twyman, a director of Deltapoll, a public opinion consultancy.

Indeed, the small size of the market is what may have alerted the authorities to unusual activity in the first place: The spike would likely not be noticed in a market like horse racing or soccer.

Britain has a curious relationship with betting, perhaps best illustrated by its place within sport. In soccer, for example, as in baseball, players are completely forbidden from betting on their own sport.

Last year, the England striker Ivan Toney was banned for six months for gambling on games. Lucas Paquetá, a Brazilian midfield player, could be banned for life if he is found guilty of gambling on games in which he was a participant. He has strenuously denied the allegations.

Both Mr. Toney and Mr. Paquetá, though, play for club teams — Brentford and West Ham, respectively — that were sponsored last season by gambling companies. They play in stadiums plastered with the logos of betting shops. And Brentford’s owner, Matthew Benham, bought the club with money he made in his hugely successful career as a professional sports gambler.

That sort of cognitive dissonance around gambling is familiar in Britain. If gambling takes place in one of the thousands of bookmakers’ shops on the country’s high streets, it is viewed as a social blight, a troubling and pernicious addiction.

If it takes place at Royal Ascot, and you are wearing a nice hat, it is the social event of the season. It was telling that Mr. Williams, the prime minister’s aide, described his wager as a “flutter” — a Britishism for a small bet, one that is inherently trivial, harmless and fun.

The election scandal has resonated with voters not because they disapprove of all gambling, experts said, but because of what it suggested about the ethics of the governing party.

“It encapsulates what everyone was already thinking,” said Mr. Twyman. “It reinforces an existing narrative that was built around the historic issues from Partygate. And it has an opportunity cost: People are talking about this, rather than what the Conservatives want them to be talking about.”

The extent to which it has cut through to ordinary people is breathtaking, according to Mr. Tryl of More in Common. Its data suggests that the betting scandal, along with Mr. Sunak’s “gaffes” around D-Day and his comments about cable TV, have become the defining topics of the campaign.

The allegations have not made much difference in the polls, but that should be scant relief for the Conservatives, Mr. Tryl said, because it did not reflect how little the public cares, but how much of the electorate had already turned against his party. “A lot of people had already gone,” he said.

That, certainly, is the bookmakers’ view: The Conservatives are currently 70/1 to retain power on July 4.