America is approaching the point where the government employee unions are so powerful that even the liberals think it’s getting out of control.

It used to be that taking on public-sector unions was the job of conservative Republicans like President Reagan, who took on the federal air traffic controllers, or Governor Scott Walker of Wisconsin, who passed a law curbing the ability of government employee unions in his state to bargain collectively.

But it’s gotten to the point where even New York’s mayor, Michael Bloomberg — a mayor who favors tightened government restrictions on guns, tobacco, trans-fats, and large bottles of non-dietetic iced tea, and who endorsed President Obama’s re-election — is drawing a line in the sand.

One sign came earlier this month, when Mr. Bloomberg likened the United Federation of Teachers, the union that represents the city’s public school teachers, to the National Rifle Association. In post-Newtown New York City, that was not either intended as a compliment or taken as one. The mayor was upset that the union had been blocking an effort to evaluate teacher performance.

But the United Federation of Teachers-National Rifle Association comparison looks like a warm-up compared to the mayor’s upcoming showdown with Local 1181 of the Amalgamated Transit Union, which represents the drivers of the yellow school buses that deliver students to the city’s schools. They are threatening a strike, which the city is warning may come as soon as this week.

The mayor’s wire service reported “The cost of bus service has skyrocketed to $1.1 billion a year from $71 million in 1979.” The mayor himself said Sunday that the cost works out to “an average of $6,900 per student; that’s far more than any other school system in the country. Los Angeles, second only to New York in size, pays just $3,100 per student.”

One can do the math onerself — at $6,900 a student, a bus carrying 30 students grosses $207,000 a year, which should be plenty to pay for a driver, a bus, gas, insurance, and some profits for the contractor. The drivers themselves aren’t city employees, but work for contractors, so the issue in their threatened strike isn’t so much the wages and benefits of the drivers but rather the city’s plan to re-bid the contracts in a court-directed way that wouldn’t protect the jobs, seniority, and benefits of the drivers if their company lost the contract. The mayor says “the new contracts will save taxpayers $95 million over five years; that’s money we are putting back into classrooms, where it’s needed most.”

When the mayor talks like that, he reminds me of Scott Walker, who tells audiences that he has children in the Wisconsin public schools and that he’s not hostile to teachers, but that he just wants to make sure that the interests of the students and the taxpayers don’t take a back seat to the interests of the union.

Mr. Bloomberg is girding for a strike, issuing MetroCards for students to take the subway or city buses instead of school buses, and offering to reimburse parents for taxis or for driving their own children at 55 cents a mile.

The issue with the school bus contractors is a caution that not all problems with government-provided services can be solved by replacing government employees with outside contractors. In recent years the school bus companies have been found to have contributed to air pollution by violating city and state laws limiting idling time, and they were involved, though not indicted, in a scandal in which seven city inspectors were convicted of accepting bribes from the bus companies.

A 2009 New York Times article reported, “In court filings in a separate case, federal investigators have said they believe that organized crime retains influence in the New York City school bus market. But they did not specify how that influence is felt except for identifying Salvatore Battaglia, the former head of the school bus drivers’ union, as an organized crime associate. In that case, Mr. Battaglia pleaded guilty to extortion and is serving a 57-month prison sentence.”

It’s hard to overstate the role of unions representing either direct government employees or government contractors in spreading the idea that rich people are to blame for all of America’s problems, which can be solved (the story goes) only by raising taxes on the rich. The home page of the Amalgamated Transit Union features a particularly nasty union-sponsored YouTube video with the claim that rich people “love their money more than anything in the whole world.”

When public-sector union power reaches the point at which even Mayor Bloomberg concludes that enough is enough, it’s a hopeful sign. If he stands firm on this school-bus driver strike, people may remember him for it long after they’ve forgotten his campaign against supersized sodas.

Mr. Stoll is editor of FutureOfCapitalism.com.