Must U.S. Pressure Israel's Electorate?

At the "National Interest," Anatol Lieven is deeply pessimistic about whether Sen. Mitchell's Middle East mission will succeed. He writes that while everyone knows what a deal between Israel and the Palestinians would look like, Israel's politics will probably keep it from making a deal:

Lacking a clear offer from Israel (as opposed to partial, qualified and revocable concessions) the PLO leadership has never had the political ammunition with which to confront its own extremists. The United States has remained throughout an essentially unconditional supporter of Israel, instead of bringing to bear the massive pressure necessary to concentrate the minds of the Israeli electorate on the real choices facing them.