Last week a visibly cold Rishi Sunak stood in front of No. 10 Downing Street in a downpour to announce the date of the general election — July 4, months earlier than expected — to an indifferent nation. “Now is the moment for Britain to choose its future,” Mr. Sunak said, as water soaked into his suit.

That it doesn’t seem to have occurred to his team to hold the event inside, or even give him an umbrella, does rather symbolize the state his Conservative Party finds itself in. Perhaps Mr. Sunak, his party now routinely polling more than 20 points behind the opposition Labour Party, has given up and wants to get it over with. Or maybe it was because another expected round of election-bribe tax cuts in September looked less plausible, given recent financial forecasts, and so the thought of grimly hanging on until the fall suddenly seemed much less attractive.

Either way, by bringing the election forward, Mr. Sunak has played his last card. This damp and deflated moment will probably be the beginning of the end for Mr. Sunak’s career in British politics, after a swift, almost dizzying rise to the top. And his legacy may be the reminder that it can be a very bad idea to get everything you want too soon.

Mr. Sunak became a member of Parliament in 2015 after a successful career in finance and publicly backed Leave in the Brexit referendum when many of the party’s up-and-comers had stayed loyal to the party leadership and backed Remain. That proved to be a smart career decision. By 2018 he had his first ministerial position, and by 2019 — after co-writing a sycophantic newspaper article for The Times of London, “The Tories Are in Deep Peril. Only Boris Johnson Can Save Us” — he was chief secretary to the Treasury in Mr. Johnson’s government. After Mr. Johnson had an explosive row with his chancellor of the Exchequer, Mr. Sunak was installed as a compliant and numerate alternative.

During the pandemic, Mr. Sunak’s dapper suits and apparent calm offered a stark contrast to Mr. Johnson’s shambolic bluster. By the time Mr. Johnson became entangled in several scandals, Mr. Sunak looked like a potential successor. That he managed to lose the leadership contest to Liz Truss should have been an early clue to his weaknesses. But then Ms. Truss set fire to her own premiership, and Mr. Sunak was quickly appointed to replace her — when his only opponent withdrew — in October 2022. At age 42, he was the youngest prime minister in more than 200 years.

The problems with this rapid rise have been apparent during his time at Downing Street. Mr. Sunak has never run a department like health or education, and he just doesn’t understand how public sector institutions work. This may explain his decision to promise to cut record-level waiting lists in the National Health Service while refusing to negotiate with striking doctors, rendering the pledge impossible. It may also shed light on his plan to deport thousands of asylum seekers to Rwanda regardless of where they came from, which has baffled anyone with government experience. Whatever one thinks about the ethics of the policy, it was just never going to work.

As the election begins in earnest, his lack of experience running a national campaign is also becoming obvious. He has struggled to rally Conservative lawmakers, particularly when so many were blindsided by the earlier date. One who had a holiday to Greece planned decided to go anyway — for a “much needed break,” he’s reported to have said.

He projects neither charm nor charisma and can come across as defensive and petulant in interviews. In response to an impassioned question about poverty on a popular daytime television show, he started speaking insistently about making it harder for children to have access to social media.

Mr. Sunak also hasn’t made it easy for voters to get a clear sense of what he stands for. One of the great ironies of this Parliament is that Mr. Sunak is ideologically to the right of Mr. Johnson, though he is often seen by the former prime minister’s fans as a centrist technocrat. Perhaps because his interests are so eclectic — he bounces around between tech utopianism about the future of A.I., tax cuts, smoking bans and reforming high school education.

This combination of a confusing agenda, inexperience and lack of basic political acumen would have been toxic at any time. But at what appears to be the end of a chaotic 14 years of Conservative rule, it has put his party in a genuinely existential position: The Tories are on track for the worst beating in their history.

This is going to be a dispiriting few weeks for Britons. The National Health Service is in a state of near collapse, several local authorities have declared municipal bankruptcy (and more are expected to follow), and British prisons are running out of space. Economic growth is sluggish. Britain needs a real conversation about its future that neither party is going to want to have.

Labour, already so far ahead, will prefer to avoid major errors and point to the failures of more than a decade of Conservative government, rather than anything significant it will do to improve voters’ lives. And the Conservatives won’t want to talk about it because, well, these are the failures of more than a decade of Conservative government. Instead they are promising, if re-elected, to revive national service for 18-year-olds and a $3 billion tax break for pensioners — straightforward pitches to older voters who might be contemplating voting for the upstart, right-wing Reform party. (And both designed to stem losses, rather than win an election.)

Brexit, which a majority of Britons now consider a failure, will also barely be mentioned. It is too unpopular for the Conservatives to claim as a success, and not yet unpopular enough for Labour to attack it without alienating Leave voters.

How Labour deals with the challenges of governing will be determined in part by the size of its majority and the space that gives it to maneuver. Mr. Sunak has, if nothing else, given it some handy lessons in what not to do. If the rumors, which he denies, are to be believed, he will leave politics after the election and return to finance, possibly in America. One suspects he’d be a lot happier.