Over the past decades, the case for enhancing union power has grown both stronger and weaker. On the one hand, as wages have stagnated while profits have soared, it does seem that there is something out of whack in the balance of power between labor and capital. Workers need some new way to collectively bargain for more money.

On the other hand, unions, and especially public-sector unions, have done a lot over the past decades to rigidify workplaces, especially government. Teachers’ unions have become the single biggest impediment to school reform. Police unions have become an impediment to police reform.

If you look at all the proposals that have been discussed since the cases of Michael Brown in Ferguson, Mo., and Eric Garner in New York, you find that somewhere or other around the country, police unions have opposed all of them:

GETTING RID OF BAD COPS A small percentage of cops commit most of the abuses. A study by WNYC News in New York found that, since 2009, 40 percent of the “resisting arrest” charges were filed by just 5 percent of New York Police Department officers. In other words, most officers rarely get in a confrontation that leads to that charge, but a few officers often get in violent confrontations.

But it’s very hard to remove the bad apples from the force. Trying to protect their members, unions have weakened accountability. The investigation process is softer on police than it would be on anyone else. In parts of the country, contract rules stipulate that officers get a 48-hour cooling-off period before having to respond to questions. They have access to the names and testimony of their accusers. They can be questioned only by one person at a time. They can’t be threatened with disciplinary action during questioning.

More seriously, cops who are punished can be reinstated through a secretive appeals process that favors job retention over public safety. In The Atlantic, Conor Friedersdorf has a riveting piece with egregious stories of cops who have returned to the force after clear incompetence. Hector Jimenez was an Oakland, Calif., cop who shot and killed an unarmed 20-year-old man in 2007. Seven months later, he killed another unarmed man, shooting him in the back three times while he ran away. The city paid damages. Jimenez was fired. But he appealed through his union and was reinstated with back pay.

CAMERAS There’s long been talk about equipping cops with wearable cameras. In Miami, Boston, and Wichita, Kan., city officials bandied about such plans, but the local unions moved to thwart them, arguing, in one case, that wearing cameras “will distract officers from their duties, and hamper their ability to act and react in dangerous situations.”

DEMILITARIZATION After riots in Ferguson, there was basically a national consensus that police don’t need mine-resistant, ambush-protected monster vehicles and military-style grenade launchers. But there’s support for the program in Washington among the defense industry and the unions. A union executive told Bloomberg News earlier this month that representatives from the Fraternal Order of Police reached out to “maybe 80 percent of senators and half the House” to defend the program. A representative of the International Union of Police Associations wrote in August after the shooting death of Brown, “I believe that law enforcement officers should have available to them any and all tools necessary to do their job and protect their community.”

STOP-AND-FRISK In New York, a court order mandated that there be federal oversight of the New York Police Department to monitor stop-and-frisk practices, a procedure that disproportionately affects minority men. The Patrolmen’s Benevolent Association moved to stall the ruling and questioned its impact. “We continue to have serious concerns about how these remedies will impact our members and the ability to do their jobs,” the president of the association said.

COMMUNITY RELATIONS In Philadelphia, a civilian oversight commission suggested that police officers apologize to citizens who complain of being mistreated. The local chief of the Fraternal Order of Police responded with a hysterical letter in March 2012 claiming that the commission was trying “to further weaken and demoralize the Philadelphia Police Department in a time of crisis with a significantly growing crime problem in this city. ... Your group poses a direct threat to public safety in this city. A threat which should no longer be tolerated by our citizens or their government.”

We get mad at racism, but most government outrages have structural roots. The left doesn’t want to go after police unions because they’re unions. The right doesn’t want to because they represent law and order. Politicians of all stripes shy away because they are powerful.

Now we have a test case to see if the people who march about the Garner case have the stamina to force change. Legitimate union advocacy has become extreme because it has gone unchecked. Most cops do hard jobs well, but right now there’s a crisis of accountability.