Sen. Bernie Sanders introduced his “Medicare for All” bill Wednesday afternoon, alongside a group of 16 cosponsors that includes some of the party’s leading contenders for 2020.

The biggest obstacle facing Sanders’ proposal is that it would be a big revenue suck. Sanders suggested that “there needs to be vigorous debate” about how to raise revenue to back it, “unlike the Republican leadership in Congress which held no hearings on their disastrous bill” that didn’t pass through Congress.

Here’s how he might pay for it:

A 7.5% income-based premium paid by employers, which his paper claims will raise $3.9 trillion over 10 years. This would exempt certain small businesses.