One of Hillary Clinton’s rivals for the 2016 Democratic presidential nomination received an informal boost Wednesday from a curious source: Mrs. Clinton’s former campaign manager, New York City Mayor Bill de Blasio.

Mr. de Blasio, who managed Mrs. Clinton’s winning 2000 campaign for Senate and supported her 2008 presidential bid, said at an unrelated news conference that “I’ve always liked what I heard from Bernie Sanders,” the independent Vermont senator who is running for the Democratic 2016 nomination.