Rosenblum grew up in Philadelphia, where he worked as a cutter in the men's clothing industry.
Rosenblum attempted to have the delegates seated, but gave up and led the majority of the union's members to found the new Amalgamated Clothing Workers of America.
In 1940, he moved to New York City to become executive vice president of the union, then in 1946, he was elected as its secretary-treasurer.
In this role, he focused on developing the union's social insurance program.
[2] He later opposed American involvement in the Vietnam War, and was active in the peace movement.