Expanded background checks may have been defeated last month in the Senate, but one area of bipartisan gun-control consensus is gathering steam in American cities: tougher sentences, including mandatory minimums, for illegal firearm possession.

Harsh proposals, backed by both gun-control advocates and Second Amendment absolutists, have been introduced in Illinois and Pennsylvania, home to the violence-plagued cities of Chicago and Philadelphia. Chicago Mayor Rahm Emanuel is campaigning for state legislation that would increase mandatory minimums for illegal gun possession from one to three years. In Pennsylvania, members of both political parties are championing a mandatory five-year-minimum sentence for any convicted felon caught carrying a gun, and now a separate bill that would apply a two-year minimum for illegal gun possession exclusively in Philadelphia.

Advocates cite New York City's three-and-a-half-year mandatory minimum sentence as a model. “The NRA believes—rightly—that enforcing the law means prosecuting criminals to the fullest extent,” New York Mayor Michael Bloomberg wrote last year. It's easy to see why such laws remain popular: Rampant gun violence persists, and city-dwellers want action. But a more cynical calculation is also at play. Politicians know the policies are ineffective but continue to vote for them lest they be painted as soft on crime. Indeed, research demonstrates that mandatory minimums create unequal and sometimes unjust sentences. They have also helped make the United States the most incarcerated nation on earth.

But the most basic reason for opposing mandatory sentences for gun possession is that mandatory minimums don’t work. You often hear this argument regarding drug crimes, which are, admittedly, less menacing to society than illegal gun possession. But research shows that mandatory minimums are ineffective whether they be applied to unlucky dope fiends or gun-toting corner boys.

"Mandatory penalty laws have not been credibly shown to have measurable deterrent effects for any save minor crimes such as speeding or illegal parking or for short-term effects that quickly waste away," writes University of Minnesota law professor Michael Tonry in the 2009 study "The Mostly Unintended Effects of Mandatory Penalties: Two Centuries of Consistent Findings."