President Obama just took a fateful step that’s near-certain to mark the start of his lame-duck days in office — the time when members of Congress don’t worry about crossing him because he’s got so little time left to take revenge.

Obama late Friday formally vetoed the Justice Against Sponsors of Terrorism Act, which would let the families of 9/11 victims sue Saudi Arabia over its alleged role in aiding the attacks. But Congress is poised to override his veto for the first time ever.

Even New York’s own Chuck Schumer, the next leader of the Senate Democrats, is on board — as is Nancy Pelosi, the leader of the House Democrats.

The New York senator called Obama’s veto “disappointing,” and said justice shouldn’t be “thrown overboard because of diplomatic concerns.”

On policy, the president makes some good points: His fears that the bill might backfire by prompting retaliation against US assets and personnel abroad are shared by such stalwart Republicans as former US Attorney General Michael Mukasey and former UN Ambassador John Bolton.

But the politics are another matter: On this, he’s fighting 9/11 families who strongly support JASTA as a path toward full accountability — and also trial lawyers, a key Democratic constituency, who drool at the prospect of vast fees from suing the Saudis.

Obama can take solace in this much: He’s made it awfully far before reaching this point. We’re just weeks from the Election Day that will choose his replacement; most presidents go lame long before this point.