Sen. Elizabeth Warren (D-Mass.)Sen. Kamala Harris (D-Calif.)
Sen. Sheldon Whitehouse (D-R.I.)
Sen. Cory Booker (D-N.J.)
Sen. Jeff Merkley (D-Ore.)
Sen. Brian Schatz (D-Hawaii)
Sen. Ed Markey (D-Mass.)
Sen. Tammy Baldwin (D-Wis.) Sen. Tim Kaine (D-Va.) Sen. Michael Bennet (D-Colo.) Sen. Angus King (I-Maine) Sen. Richard Blumenthal (D-Conn.) Sen. Amy Klobuchar (D-Minn.) Sen. Sherrod Brown (D-Ohio) Sen. Patrick Leahy (D-Vt.) Sen. Maria Cantwell (D-Wash.) Sen. Joe Manchin (D-W.V.) Sen. Ben Cardin (D-Md.) Sen. Claire McCaskill (D-Mo.) Sen. Tom Carper (D-Del.) Sen. Robert Menendez (D-N.J.) Sen. Bob Casey (D-Pa.) Sen. Chris Murphy (D-Conn.) Sen. Chris Coons (D-Del.) Sen. Patty Murray (D-Wash.) Sen. Catherine Cortez Masto (D-Nev.) Sen. Bill Nelson (D-Fla.) Sen. Joe Donnelly (D-Ind.) Sen. Gary Peters (D-Mich.) Sen. Tammy Duckworth (D-Ill.) Sen. Jack Reed (D-R.I.) Sen. Dick Durbin (D-Ill.) Sen. Chuck Schumer (D-N.Y.) Sen. Dianne Feinstein (D-Calif.) Sen. Jeanne Shaheen (D-N.H.) Sen. Al Franken (D-Minn.) Sen. Debbie Stabenow (D-Mich.) Sen. Kirsten Gillibrand (D-N.Y.) Sen. Jon Tester (D-Mont.) Sen. Maggie Hassan (D-N.H.) Sen. Tom Udall (D-N.M.) Sen. Martin Heinreich (D-N.M.) Sen. Chris Van Hollen (D-Md.) Sen. Heidi Heitkamp (D-N.D.) Sen. Mark Warner (D-Va.) Sen. Mazie Hirono (D-Hawaii) Sen. Ron Wyden (D-Ore.)
Over time, some issues become so widely accepted within a party as to be a de facto requirement for anyone aspiring to lead it. No Democrat would run for president, or even for House or Senate minority leader, without supporting the DREAM Act. No Republican would try for a leadership position without supporting repeal of the estate tax.
And the way things are going, soon no Democratic leader will be able to oppose single-payer.