Bernie Sanders admits he wants to raise taxes across the board – an unusual statement in presidential politics. (Peter Stevenson/The Washington Post)

There’s one simple rule that presidential candidates from both major parties tend to follow: Don’t suggest raising taxes, especially on the middle class.

But Bernie Sanders did just that at Monday’s CNN town hall in Des Moines, Iowa. It was always clear that Sanders’s plans to increased social security payments, free college tuition and universal health care would be expensive. Critics said those plans could never be paid for without huge tax increases. And Sanders acknowledged as much in Des Moines, giving his opponents a soundbite tailor-made for attack ads.

“We will raise taxes,” he admitted. “Yes, we will.”

Granted, he argued that the tax increases he’s proposing would actually save taxpayers money in the long term – for example, by eliminating premiums for private health insurance. But will that make tax increases more palatable to voters, or his political opponents?

Sanders isn’t the first presidential candidate to propose higher taxes for at least some Americans. President Obama likes to say wealthy Americans should “pay their fare share,” while Hillary Clinton recently came out in favor of the “Buffett rule,” which says the wealthy should pay the same tax rate as middle-income Americans. The big difference with Sanders: His plan would likely raise taxes for Americans across the board.

What Sanders may be forgetting, though, is the last Democratic nominee to suggest raising taxes, Walter Mondale. In his acceptance speech at the 1984 Democratic National Convention, Mondale was very blunt about his plan, saying, “Mr. Reagan will raise taxes. So will I. He won’t tell you. I just did.” Mondale went on to win just 13 electoral votes – from the District of Columbia and his home state of Minnesota -- while Reagan swept up every other state. It was the worst loss by a Democrat in the history of presidential elections.

Raising taxes isn’t popular with voters and isn’t popular with Congress, either. Just about every congressional Republicans signs Grover Norquist’s “Taxpayer Protection Pledge,” promising to oppose any tax hikes on businesses or individuals.

And that is the core of Sanders’s problem: Even if he can get over the considerable hurdle of selling his voters on the plan, he probably won't be able to get Congress to sign off on big tax increases.