J Street didn't make it into the Conference of Presidents. The Elders of Zion just would not have it. Not that I think the political position of J Street carries a banner of mainstream Judaism, but neither does Alpha Epsilon Pi Jewish College fraternity which had a role in blackballing them.
Silencing or stonewalling opposition, the implementation of technique or sometimes force to deny Jews their seat at the table has been a work in progress for some time. Certainly at the synagogue or Federation level there have been people who count from the USY cliques to the macher swoops. One may be convenient or inconvenient, but not inherently important in your own right. The market place of ideas that has made Judaism thrive can be too restrictive, as the Conservatives of the past half century should have learned from the Pew Research Study outcome. Once excluded with a safe landing elsewhere, people do not necessarily desire re-entry.