I was just listening to a Marc Shapiro class where he discussed the monumental proclaimation by serveral leading rabbanim in the 1950s against Orthodox groups being members of the Synogogue Council of America, an interdenominational Jewish umbrella organization, as well as other such groups. R' Soloveitchik and R' Eliezer Silver famously did not sign on to the letter, possibly for different reasons. Here is a rabbi who worked with other Jewish denominations - bless him. (Working as an individual with other congregtations was not necessarily addressed in the proclaimation.)
Rabbi had a passion for faith, entire community - linked article, Denver Post
"More than 1,200 people gathered Tuesday afternoon to honor Rabbi Daniel Goldberger, one of Denver's most respected and best-known rabbis.
Goldberger, who died Monday at age 83 after years of failing health, was eulogized as a "giant of faith and a great man" by Rabbi Stanley Wagner, a longtime friend."
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