We must make our choice. We may have democracy, or we may have wealth concentrated in the hands of a few, but we can’t have both.

Louis Brandeis (1856-1941) American lawyer, activist, Supreme Court Justice (1916-39)

Quoted in Raymond Lonergan, “A Steadfast Friend of Labor,” Labor magazine, Washington, D.C. (1941-10-14)
    (Source)

Brandeis had died on the 5th of the month. Lonergan, himself, gives the quote second hand ("... he once said to a younger friend, who appreciated the opportunity to sit at the feet of this modern Gamaliel"). This remains the only citation of the quotation, on those rare occasions when it is given such.
Collected in Irving Dillard, ed., Mr. Justice Brandeis, Great American (1941).