National Nurses United, the country's biggest nurses' union, will endorse Bernie Sanders in the 2020 presidential race this week, the New York Times reports.

Why it matters: It's significant that the group chose to back Sanders and his "Medicare for All" plan despite the fact that he's facing a progressive challenger in Elizabeth Warren on the issue.

The union also endorsed Sanders during the 2016 presidential election.

The decision will give Sanders millions in financial backing from the union's super PAC — despite the fact that the candidate, along with the Democratic field's more progressive members, have largely disavowed such support.

The big picture: Sanders has accumulated other endorsements from labor unions, including the United Electrical, Radio and Machine Workers of America and the National Union of Healthcare Workers (which backed both him and Warren).

Go deeper: The quiet race to win Hillary Clinton's endorsement