Democratic presidential candidates Elizabeth Warren and Bernie Sanders want to enact a wealth tax to pay for their large government programs. The biggest barrier may not be Congress, but the Constitution.

The Constitution says the federal government is prohibited from imposing “direct taxes,” except for income tax, without distributing the money among the states according to population. The progressive candidates’ wealth taxes would be used for federal programs such as free health care, free public-college tuition and universal day care.

Even if Democrats manage to win control of the House, the Senate and the White House, wealth-tax supporters have become increasingly worried that their policy plans could be stymied by a constitutional challenge. This was a concern even before President Donald Trump installed two new members of the Supreme Court.

So left-leaning tax experts are working on ways to put a wealth tax on a more solid constitutional footing. And there is no shortage of ideas.