Eventually, someone was going to do the math and then progressive Democrats would have to explain their support for a program that would eat up almost all federal revenue each year.

Now, a new study from the libertarian Mercatus Center finds that the "Medicare for all" plan introduced by Sen. Bernie Sanders, I-Vt., would increase government spending on healthcare by $32.6 trillion over the next decade assuming it is successful in most of its stated goals. That’s about $3.2 trillion each year, an ungodly amount compared to the $3.3 trillion the federal government takes in annually in revenue. If hospitals and doctors refuse to accept less money for their services, the plan could prove far more expensive, or else the quality of care could suffer dramatically, or else there will be some result combining those two undesirable outcomes.

Sanders couldn’t refute that economic analysis when asked to defend his plan. He could only quibble with the authors by maligning the Mercatus report as “the Koch brothers response to the growing support in our country for a ‘Medicare for all’ program.”

This simply won’t do. One, because other more liberal think tanks, like the Urban Institute, have made similar projections. And two, because already a third of the Democratic Party has endorsed the idea without actually taking a good hard look at what it would cost. When asked whether he had his own numbers, even Sanders admitted his office never bothered to commission a cost analysis.

That leaves progressives with little political cover. One-third of Democratic senators, 16 in total, co-sponsored Medicare-for-All legislation, and 60 percent of House Democrats, 123 to be exact, did the same. Six of those Senate Democrats, like Sen. Tammy Baldwin of Wisconsin, are up for re-election in 2018 and at least three are presumed 2020 presidential candidates like Sens. Cory Booker of New Jersey, Kamala Harris of California, and Elizabeth Warren of Massachusetts.

Each must be pushed past the soundbites to give an honest explanation of where the federal government can possibly find the money for such a thing. And while preparing, they should look to Alexandria Ocasio-Cortez’s example.

Although Ocasio-Cortez caught plenty of deserved-flak for flubbing an answer about paying for a socialist agenda that includes "Medicare for all," she at least tried all the usual answer about wealthy people and rich corporations “paying their fair share.” She laid out wishful accounting about what closing tax loopholes and creating carbon taxes could actually raise. She even pointed to imaginary defense spending numbers to erroneously claim that the military received “a $700 billion budget increase,” by way of suggesting potential cuts.

According to the Mercatus analysis, though, new taxes and budget reshuffling just won't be enough. The federal government could double individual and corporate income tax revenues and they still wouldn’t have enough to fund "Medicare for all" on top of the rest of government expenses. It seems that Ocasio-Cortez, like her more senior progressive peers, are guilty of wishful economic thinking.

The idea sounds good, but the math just doesn’t work. Democrats don’t have a good answer as to why.