Once upon a time, Kirsten Gillibrand was a great friend of Wall Street, and she reaped the rewards in the form of hefty donations from the likes of Goldman Sachs and JPMorgan. But, oh, how she’s “grown.”

Now the senator is jumping onto Bernie Sanders’ ship in calling for a tax meant to raise billions off the financial industry.

The .03 percent tax on every securities transaction is just the latest version of an idea that’s been kicking around for decades, called “the Robin Hood tax” since it aims to take from the rich to give to the poor.

In fact, it would give to the government — and take from plenty of folks who aren’t rich, hitting pension funds and 401(k) accounts just as hard as the fat-cats.

And at the cost of pushing everyone to do their transactions somewhere else. It wouldn’t be the end of Wall Street — but it would be another step toward pushing big banks and trading firms to cut back here and expand in other financial capitals.

More, as a 2011 study by the Congressional Budget Office found, it “would raise the cost of financing new investment,” making it harder for non-financial businesses to grow.

Plus, as everyone knows, once politicians start taxing something, they’re guaranteed to sooner or later tax it more: The federal income tax originally only hit the super-rich.

Of course, the whole thing is a nonstarter in Congress for now, appealing only to left-leaning Democrats. But that’s all the senator is about these days, with her eye on the party’s 2020 nomination.

Gillibrand says the tax is needed because “income inequality” is “the greatest risk we have to our democracy now.” Most Americans see much bigger threats to the Republic — including a glut of politicians all too eager to appeal to ideological extremes, even if it means targeting one of the most important industries in their own state.