What convention will bring together 4,000 highly influential men and women who base their worth on fairness, trust and the ability to clearly articulate the issues of the day? And why don't Al Gore and George W. Bush want to speak to any of them? The answer to the first question is the Radio-Television News Directors Association, which meets in Minneapolis Sept. 13-16. And to the second? Both presidential candidates declined invitations from RTNDA President Barbara Cochran, in large part because the land of liberal politics and frozen food is not exactly a key state. "If Al Gore has to campaign in Minnesota after Labor Day," Cochran quipped, "he's in a lot of trouble." One politician is interested: Minnesota Gov. Jesse Ventura is expected to be there, and to hand an RTNDA award to fellow Minnesotan and broadcast pioneer Stanley Hubbard. For the record, the last presidential candidate to talk to the group in person was that great friend of the press, Richard M. Nixon.