Washington — Watch your back, Donald Trump. Mike Bloomberg is opening his big, fat wallet.

The former New York City mayor’s Independence USA PAC is airing a TV ad slamming New Hampshire Republican Sen. Kelly Ayotte for her support of Trump.

“Independent Republicans all over the nation are breaking with Donald Trump and disavowing him,” PAC spokesman Marc La Vorgna told reporters.

“Unfortunately, instead of demonstrating the independence she claims, Kelly Ayotte is still supporting Trump. That’s not the kind of leadership New Hampshire needs.”

It’s a $2 million ad buy running in Boston and Manchester markets.

The goal of the ad is “To ensure Maggie Hassan is the next US senator,” Bloomberg aide Howard Wolfson, a former Hillary Clinton aide, told The Post.

“Kelly Ayotte voted against the bipartisan Manchin-Toomey bill … we’re committed to helping elect people to the Senate who will pursue sensible gun policy.”

Bloomberg endorsed Democrat Clinton and eviscerated Trump in a speech at the DNC convention last month.

This first ad buy may be just the beginning of the billionaire former mayor’s attempt to hang Trump around the neck of other GOP candidates’ necks.

But Wolfson isn’t showing his card. “We’re not spending any money on the presidential race,” Wolfson said of the present moment.

“I don’t discuss what we may or may not do going further. And we have not thus far and are not currently.”