Days before liberal activist Allard Lowenstein was shot dead in his New York office last month, he told close friends he intended to run for the coveted House seat from Manhattan's affluent, sophisticated and wealthy East Side. That seat is currently held by Republican Rep. Bill Green, an heir to the Grand Union fortune.

But this fall there may be two Greens (unrelated) on the ballot. Mark Green, Ralph Nader's director of Public Citizen Watch, is expected to run for the seat once held by, among others, John Lindsay and Ed Koch.

"You've caught me during my Hamlet-like period," Mark Green said two and a half weeks ago, as he pondered whether to give up his visible post in exchange for an uncertain political race. Friends expect him to make the bid, though Green says Nader thinks he's crazy. "Ralph regards what we do so highly that he can't imagine surrendering this unique place in public life for being one of 435 people," says Green, who edited the critical paperback Who Runs Congress?

In a tragic twist, Mark Green received a phone call from Lowenstein an hour before Lowenstein was murdered. Green was supposed to have met with him later that afternoon, at which time Lowenstein intended to tell Green he intended to run for Bill Green's seat.

Footnote: Should both Greens be candidates, Mark Green will appear first on the ballot since the congressman's full name is S. William Green. That favors Mark Green, who says on that matter he has "no comment -- ask my father about that."