As Bob Dylan told us long ago, “You don’t need a weatherman to know which way the wind blows.” These days, a foul wind blows from City Hall.

With subpoenas issued, investigations spreading and at least one federal grand jury at work, the question isn’t whether Bill de Blasio’s administration has a corruption problem. The questions are how big is the problem, how many agencies are tainted and how high up the pecking order does it go?

The preliminary answers are worrisome. A small army of investigators, including the FBI, is on the hunt, and all roads appear to lead straight to Mayor Putz.

The two biggest and most recent scandals involve lucrative gifts and cash given to current and former top officers in the NYPD, and the removal of a deed restriction and sale of a city-owned Manhattan building that netted private investors a cool $72 million profit.

The rub is that both cases feature donors who contributed large sums to the mayor’s campaign and his slush fund. That would suggest the fish stinks from the head.

That pattern isn’t new. There were red flags when de Blasio went to extraordinary lengths, including the approval of huge raises for the City Council, to do the bidding of donors who demanded the end of the beloved horse carriages. He also used phony traffic statistics to try to crush Uber, a move that would have meant a windfall to the yellow medallion kings who donated more than $500,000 to the mayor.

The possibility that de Blasio presides over a pervasive pay-to-play culture cannot be dismissed. To cite yet another example, the teachers union piled cash into his political fund, and came away with an excessively generous contract.

All those suspect exchanges demand the strictest scrutiny because even a suggestion that City Hall is for sale is intolerable. As the incidents multiply, with the mayor’s city and state Democratic allies joining the probes, we are long past the suggestion threshold. A little more than two years into de Blasio’s term, an integrity reckoning is overdue.

Yet as bad as the corruption problem is, it is only part of a larger failing. From the moment he announced his 2013 campaign, de Blasio never pretended to speak for the whole city.

In his mind, he had sliced and diced 8 million New Yorkers into factions. Race and ethnicity here, class and geography there. It was always us against them, with “us” being anyone who had a grievance and supported him, while “them” were critics and supporters of others.

The police were a favorite target, Al Sharpton a favored friend. Outgoing Mayor Michael Bloomberg was accused of running a plantation — a plantation! — at de Blasio’s inauguration, and the crass new mayor pronounced himself untroubled.

If the ominous class and race warfare had stopped there, it would have been bad enough. But it didn’t stop.

Nor did the mayor’s obsession with politics. He fancies himself the leader of a national movement, and spends an inordinate amount of time out of the city spreading the news of his greatness. In one telling interview, he complained that people outside New York better understood him than did the benighted folk in the five boroughs.

Yet national pols aren’t impressed with him either. He had to cancel a planned forum in Iowa when none of the candidates would attend, and foolishly trekked to the cornfields there to make amends to Hillary Clinton after he snubbed her. She refused to see him, the smartest thing she’s done in years.

Back at the ranch, problems were growing in de Blasio’s absence, but even when he was at his desk, he wasn’t much interested in the vexing details of governing. When The Post pointed out the rising tide of homelessness, the mayor insisted it wasn’t true.

By the time he finally got around to conceding a problem that the whole world saw, a deputy mayor hired with great promise abandoned ship. She complained, friends said, that de Blasio had no time for her or for a substantive discussion, and was only interested in — you guessed it — the politics.

How would homelessness play? Would it hurt him? How could he use it to demonstrate his progressive values?

So it goes, day in, day out, where every event, every statement, every mayoral utterance is a horn-blowing exercise designed to boost No. 1’s political standing.

Whether it’s good for anyone in New York is an afterthought, if it’s a thought at all.

A mayor this cynical and reckless usually ends up on thin ice, and once the ice starts to break, the trouble doesn’t stop. That’s the new phase we’ve entered, as revealed by the foul wind blowing from City Hall.

Of course, this isn’t the Mayor’s first scandal: