An element of the New York Police Department’s stop-and-frisk practice was deemed unconstitutional by a federal judge on Tuesday, a ruling that may have broad implications for the city’s widespread use of police stops as a crime-fighting tactic.

The decision, the first federal ruling to find that the practice under the Bloomberg administration violates the Fourth Amendment protection against unreasonable search and seizure, focused on police stops conducted in front of several thousand private residential buildings in the Bronx enrolled in the Trespass Affidavit Program. Property managers in that program have asked the police to patrol their buildings and to arrest trespassers.

But the judge, Shira A. Scheindlin of Federal District Court in Manhattan, said officers were routinely stopping people outside the buildings without reasonable suspicion that they were trespassing.

How much blood will federal Judge Shira Scheindlin have on her hands when she finishes dismantling the most effective anti-gun-violence program in urban America?

The Manhattan jurist yesterday ordered an immediate halt to the use of stop-and-frisk against suspected trespassers outside privately owned Bronx buildings.

This, even as the buildings’ landlords have given cops permission to do so under the city’s “Clean Halls” program.

Scheindlin makes no bones about why she’s undercutting a program that has saved thousands of lives.

“The public interest in liberty and dignity under the Fourth Amendment trumps whatever modicum of added safety might be gained” by the use of stop-and-frisk, she wrote in a 157-page opinion.

Indeed, this is the same loony-left judge who refused to allow an expert to even testify about the effectiveness of stop-and-frisk as “irrelevant” and “too speculative.”

“The issue . . . is not whether trespass stops . . . [effectively] reduce crime, but whether they are constitutional,” she wrote.

Such slavish obeisance to abstraction doubtless will earn her rave reviews from New Yorkers who live in neighborhoods that are rarely subject to random shootings.

But there is no dignity in lying bleeding in the streets, shot down for a jacket or a wallet or in response to some random perceived insult.

And the fact is, thanks to stop-and-frisk, New York is not in the grip of the murder epidemic that is paralyzing Chicago, Detroit and Philadelphia — none of which have comparable programs.

Homicides here have fallen to all-time lows — reflecting the NYPD’s critical shift from simply responding to crimes in favor of proactively preventing crime through crime-targeted vigilance.