Our Revolution, accused of dark-money spending for Sanders, took only six donations over $5,000 last year, none larger than $25,000

RIVAL PRESIDENTIAL CANDIDATES have been attacking Sen. Bernie Sanders over the advocacy groups boosting his campaign, accusing the Democratic frontrunner of taking untraceable dark money and contributions from super PACs and from the nonprofit he founded in 2016, Our Revolution.

The problem with the charge: It’s not dark money, and it’s not big, either.

The issue arose most recently during the Nevada debate, when Sen. Elizabeth Warren said that everyone onstage besides her and Sen. Amy Klobuchar had taken money from super PACs, which can take in and spend money...