Support for Medicare for All is showing signs of slipping as top-tier Democratic presidential candidates Sens. Elizabeth Warren and Bernie Sanders struggle to sell the proposal without providing specifics on the financial costs for voters.

Meanwhile, an abundance of other health proposals aired by the Democratic primary field is complicating messaging efforts as polls show many voters still don’t understand what specific plans would do. Fifty-one percent of Americans back the Medicare-for-All proposals, which is down 5 percentage...