FOR decades now, conservatives have pressed the case that public sector unions do not serve the common good.

The argument is philosophical and practical at once. First, the state monopoly on certain vital services makes even work slowdowns unacceptable and the ability to fire poor-performing personnel essential, and a unionized work force creates problems on both fronts.

Second, the government’s money is not its own, so negotiations between politicians and their employees (who are also often their political supporters) amount to a division of spoils rather than a sharing of profits. Third, these negotiations inevitably drive up the cost of public services, benefiting middle-class bureaucrats at the expense of the poor, and saddling governments with long-term fiscal burdens that the terms of union contracts make it extremely difficult to lift.

Finally, union lobbying power can bias public-policy decisions toward the interests of state employees. To take just one particularly perverse example: In California over the last few decades, the correctional officers union first lobbied for a prison-building spree and then, well-entrenched, exercised veto power over criminal justice reform.

These points add up to a strong argument that the rise of public sector unions represents a decadent phase in the history of the welfare state, a case study in the warping influence of self-dealing and interest-group politics.

But as we’ve been reminded by the agony of Baltimore, this argument also applies to a unionized public work force that conservatives are often loath to criticize: the police.

Police unions do have critics on the right. But thanks to a mix of cultural affinity, conservative support for law-and-order policies and police union support for Republican politicians, there hasn’t been a strong right-of-center constituency for taking on their privileges. Instead, many Republican governors have deliberately exempted police unions from collective-bargaining reforms --- and one who didn’t, John Kasich of Ohio, saw those reforms defeated.

In an irony typical of politics, then, the right’s intellectual critique of public-sector unions is illustrated by the ease with which police unions have bridled and ridden actual right-wing politicians. Which in turn has left those unions in a politically enviable position, insulated from any real pressure to reform.

Yet reform is what they need. There are many similarities between police officers and teachers: Both belong to professions filled with heroic and dedicated public servants, and both enjoy deep reservoirs of public sympathy as a result. But in both professions, unions have consistently exploited that sympathy to protect failed policies and incompetent personnel.

With this important difference, however: Even with the worst teacher, the effects are diffused across many years and many kids, and it’s hard for just one teacher to do that much damage to any given student. A bad cop, on the other hand, can leave his victim dead or permanently damaged, and under the right circumstances one cop’s bad call --- or a group of cops’ habitual thuggishness --- can be the spark that leaves a city like Baltimore in flames.

Last December, my colleague David Brooks noted that police unions are resisting change on every issue where police reform might be contemplated, from body cameras for officers to reversing the militarization of local law enforcement. But after the untimely death of Freddie Gray, no issue looms larger than the need to discipline, suspend and fire police officers who don’t belong on the streets --- and the obstacles their unions put up to that all-too-necessary process.

The cases from all over the country where unions and arbitration boards have reinstated abusive cops make for an extraordinary and depressing litany. Baltimore is no exception. Last fall, The Baltimore Sun reported on the police commissioner’s struggle to negotiate enough authority to quickly remove and punish his own cops, and the union’s resistance to swift action and real oversight persists.

What we know so far about the officer who first pursued Mr. Gray (his history of mental health issues, in particular) suggests that he might have benefited from being eased into a different line of work. This issue is particularly pressing if you believe that some of the aggressive police tactics criticized in the wake of Mr. Gray’s death, and Eric Garner’s in Staten Island --- the stress on quality-of-life and “broken windows” policing, the focus on misdemeanors and disorderly conduct --- have played a significant role in America’s declining crime rate and our much-safer cities.

Some liberals have decided these tactics haven’t made a difference, or that they aren’t needed anymore. I think this view is naive, and dangerously so. But to sustain this kind of police work, it’s necessary to restrain the excesses associated with it; to restrain those excesses, it’s necessary to hold cops accountable. And that can only happen if we reckon with the ways in which police unions, no less than other interest groups, can align against the public that their members vow to serve.