Campaign season is starting earlier and earlier, with ever more candidates throwing their hats in the ring, even though Election Day is far away.

I’m speaking, of course, of the 2021 New York City mayoral race, which ­already boasts a crowded Democratic field — even though there are more than 950 days until the primary.

This week, City Council Speaker Corey Johnson announced that he is “preparing” for a possible run — translation: He’s running — and started soliciting small donations, vowing not to accept contributions over $250.

Comptroller Scott Stringer — who has more than $2 million banked in a campaign war chest — has announced that he will begin holding “thought-raisers” in supportive living rooms around the city in order to “elevate the conversation.” Translation: He’s running.

Bronx Borough President Ruben Diaz Jr. has made his candidacy official. Eric Adams, Brooklyn Borough president, is considered likely to step in, too. Each is popular in his borough and formerly served in the Legislature, though neither has much of a citywide profile.

Striving to distinguish himself from the crowd, Johnson has vowed to reject real estate and lobbying money, the lifeblood of New York City politics. This effort to remake himself as a populist is belied somewhat by his fundraising for the 2017 cycle, when he was happy to take large checks from billionaires, real estate developers and hoteliers.

But at a time when the Democratic base appears to have lurched left, Johnson’s crafty tack may pay off.

As comptroller, Stringer has indefatigably trod his way around the city, attending as many outer-borough church functions and Midtown chicken-dinner award ceremonies as humanly possible, and is a regular at Al Sharpton’s Saturday morning mustering of the troops.

His office — tasked with auditing the city’s finances — issues report upon report about minority business ownership, marijuana legalization, affordable housing and any other issue that can give Stringer a platform to appear in touch.

Stringer and Johnson, both from the West Side of Manhattan, will face a tough fight to win their own home territory, a battle most politicians prefer to have settled in advance. And the big question is how well either can perform in the boroughs.

Stringer has won citywide ­before, but against thin competition. Johnson, who is gay, could face headwinds in socially conservative precincts in Brooklyn and the Bronx.

Eric Adams is a former cop and state senator who takes a refreshingly strong law-and-order stance, especially at a time when much of the city’s leadership seems to think that shutting down jails will eliminate crime.

But Adams has never faced a major competitive election, has flip-flopped on admission tests to the city’s selective high schools and faced pay-to-play questions about a dodgy nonprofit he set up. Adams is quirky: An avid biker and vocal vegan, he also promised to bring his gun to church after the Pittsburgh synagogue massacre.

Diaz has kept as busy as a borough president can be since declaring his candidacy and seems to be playing a long game. His terms in office have been quiet but respectable, even while corruption scandals rocked the Bronx. But with the 2018 rejection of local powerbrokers Jeff Klein and Joe Crowley, the old Bronx political machine appears to be in a state of ­realignment, and the chips could land anywhere.

One other plausible candidate is former comptroller and current state Sen. John Liu, whose 2013 mayoral run was sidetracked by a campaign finance scandal that, compared to Mayor Bill de Blasio’s shenanigans, now seems as grave as passing a ­Canadian dime at the newsstand. Liu is relentlessly ambitious and experienced, and he could disrupt the existing political calculus if he decides to run.

Ultimately, term limits are dictating the accelerated political schedule; with so many officials looking at unemployment when their current terms expire, campaign mania is contagious. So we as New York City voters are likely to be the captive audience for spiraling one-upmanship, as each candidate tries to outdo the others with radical proposals and absurd stunts.

Unfortunately, that means that the real problems we face — crumbling public housing, homelessness and treatment for the seriously mentally ill — will serve for the next two years as fodder for applause lines, ­instead of receiving the serious, competent governance they deserve.

Seth Barron is associate editor of City Journal and director of the NYC Initiative at the Manhattan Institute.