Former Green party presidential candidate Ralph Nader says that Sens. Elizabeth Warren (D., Mass.) and Bernie Sanders (Socialist, Vt.), would make a “very powerful ticket” for Democrats in the 2020 general election. Mr. Nader adds that such a combination, combined with aggressive Democratic party efforts to keep Greens off the ballot, would make it hard for a Green party candidate to attract many votes in November of next year.

Mr. Nader, who first gained fame as an anti-corporate activist in the 1960s, calls Sens. Warren...