THE RATCHET EFFECT: Sanders’ single-payer push splits Democrats.

Wisconsin Sen. Tammy Baldwin on Tuesday became the single-payer bill’s first supporter from the class of Senate Democrats up for reelection next year in states Trump carried. But other politically imperiled incumbent Democrats have said no to Sanders.

Sen. Claire McCaskill said in a brief interview that lawmakers have more work to do to keep health care costs in check “before we would think about expanding that [Medicare] system for everyone.”

Single-payer on a national level would have “a lot of problems,” McCaskill added, although she came out in support of allowing individuals as young as 55 to buy into Medicare. That idea is also backed by Baldwin and two other red-state Democrats up for reelection next year who are declining to endorse Sanders’ bill: Sens. Sherrod Brown of Ohio and Debbie Stabenow of Michigan.

Stabenow, also a member of Democratic leadership, said Tuesday that she would keep working on her Medicare-at-55 plan “because I think there is some bipartisan interest in that.” She said the party’s first order of business should be shoring up the Obamacare markets, followed by other goals.

That’s this year. By 2020, support for single payer will likely be the Democrats’ new litmus test.