Jonathan Zimmerman is a professor of history and education at New York University. He is the author of “Small Wonder: The Little Red Schoolhouse in History and Memory” (Yale University Press).

Bloomberg is buying another term in office! It’s an outrage!

That’s what lots of my fellow liberals said when billionaire Michael Bloomberg spent $102 million of his own cash – about $174 per vote – to win re-election as New York mayor in 2009. And they were right. Unchecked money turns politics into a corrupt poker game, where the well-to-do get to stack the deck.

So why aren’t these same critics complaining, now that Mr. Bloomberg is showering his millions on candidates who back gun control and same-sex marriage? Because liberals like these causes, of course.

As President Obama heads to Colorado today to praise the state’s newly passed gun-control legislation and push for action in Congress, it’s worth taking a step back to look at the money in the fight. The National Rifle Association spent roughly $25 million – more than twice as much as Bloomberg did – in the 2012 elections. Liberals will argue they’re only trying to even the playing field – to fight fire with fire, using their own loaded benefactor to counter the muscle of the NRA....