Speaking truth to power comes at a cost: Mayor Eric Adams plainly got passed over for a coveted speaking role at this week’s Democratic National Convention because he stood up to the White House on the migrant crisis.

The mayor landed on the Biden-Harris “no fly” list long months ago, when he called out the administration for turning its back on New York amid a record inundation of “asylum seekers,” thundering that “the city is being destroyed by the migrant crisis.”

He asked for help with the vast costs (now over $5 billion) and urged a “decompression” strategy that would slow the surge to the Big Apple and other hard-pressed cities — and so embarrassed the president who’d opened the floodgates and the “border czar” veep who delivered zero results beyond a handful of feckless photo-ops.

A month after the mayor spoke out, Biden dropped Adams from his campaign leadership team, while retaining far lower-profile New York leaders like Queens Rep. Grace Meng.

And now he’s been snubbed at the convention, even as LA’s Karen Bass — the mayor of a city half Gotham’s population, who took office far more recently — landed a speaking slot, and then-Mayor Bill de Blasio got one in 2016, at the last in-person convention.

Oh, and both AOC and Gov. Hochul got speeches, too.

Adams is taking it in stride, though he skipped the first two days in Chicago and was conspicuously absent from the New York delegation during Tuesday’s ceremonial Roll Call of the States.

But so much for all those other DNC speeches about the party being a “big tent” and open to debate.

Anita Dunn, the longtime top Joe Biden adviser who last week left the White House now that Joe’s left the race, revealed how things really roll in farewell remarks to junior staff last Friday.

Follow along with The Post’s live reporting of the Democratic National Convention in Chicago.

To understand the use of power in Washington, she reportedly advised, watch “The Godfather” — and recall the classic line, “Revenge is a dish best served cold.”

Payback can take take months, even years, she emphasized.

You have to wonder if that thinking is tied to the protracted federal probe of Adams’ 2021 campaign finances, including a grand jury subpoena in July atop FBI agents publicly seizing his electronic devices last year.

That can’t help but harm the mayor’s re-election effort next year, punishment for his migrant apostasy far harsher than the DNC snub.

Adams deserves the gratitude and respect of all New Yorkers for for putting his duty to them above blind loyalty to Biden and Harris.

In the runup to what’ll surely be a close election, the Harris folks who wound up running the convention could have offered an olive branch to the mayor:

Why not maximize party unity, even if you don’t think you need any help in carrying New York?

Mafia rules aren’t the only way to get ahead in this world; another truth of politics remains: “What goes around comes around.”