Cory Booker backs single-payer

Add Sen. Cory Booker to the list of prominent Democrats endorsing a single-payer health care system. Booker said in a television interview Monday that he would cosponsor Sen. Bernie Sanders's single-payer bill — which Sens. Kamala Harris and Elizabeth Warren have also endorsed. Sanders is set to introduce the bill Wednesday.

Why it matters: Support for single-payer is rapidly becoming a litmus test for Democrats who are even thinking about their party's 2020 presidential primary.