When New York’s entire political class lines up on one side of an argument, you can bet they’ve chosen the wrong side. We’re seeing an example right now in the howling directed at Extell for a plan to construct separate entrances for high- and low-income earners at a new luxury building going up on the Upper West Side.

The tower is at 40 Riverside Blvd. Under the terms of their deal, the builders got tax breaks in exchange for making 20 percent of the units affordable housing. These units will be on the lower floors, and access will be via a separate entrance everyone is now calling the “poor door.”

That, in turn, has been followed by the predictable outrage. Manhattan City Councilman Robert Jackson introduced legislation to stop it. Local state Assembly member Linda Rosenthal calls it a “blatant attempt to segregate people” that has “no place in the 21st century, let alone on the Upper West Side.” Others liken it to the servants’ entrance in Downton Abbey.

Here’s the truth about segregating the poor: That’s what almost all government welfare is about. Whether it’s public housing or failing public schools, government assistance to the poor tends to keeps them out of the market and trapped in their own separate enclaves. The only difference in this case is that the segregation is more obvious because it happens to be in the same building.

If the city or state really wants to fix this, give people who need housing assistance vouchers. That would do two good things: Give the poor more control over where they want to live, and end the need for special tax breaks for builders.