The failure of the American-sponsored Middle East peace plan could push Israelis and Palestinians over the edge of a cliff. As a result of the devastating suicide bus bombing in Jerusalem on Tuesday, that cliff seems to be getting closer by the hour. Yesterday, an Israeli helicopter gunship killed a top leader of Hamas, the terrorist group that carried out the bus attack, leading Hamas and Islamic Jihad to declare that their seven-week-old cease-fire with Israel was over. Israeli troops moved into Nablus, Hamas rockets were fired into Israel, and militants were calling for the resignation and self-exile of the Palestinian prime minister, Mahmoud Abbas.

If the peace process — and Mr. Abbas's government — is to survive, Washington needs to redouble its efforts, and both Mr. Abbas and Ariel Sharon, the Israeli prime minister, must realize that blaming the other will not suffice. Mr. Abbas had been hoping that the cease-fire would permit him to escape the difficult step of confronting Hamas and other radical groups. But these groups have only one goal — the destruction of the Jewish state — and need to be dealt with accordingly.

That said, Israel's assassination of one of Hamas's leaders seems counterproductive. Mr. Abbas and his security chief, Muhammad Dahlan, were clearly shaken by the terrorist attack in Jerusalem, and they had vowed to pursue the perpetrators. By taking pre-emptive action, the Israelis not only gave Hamas an excuse to rouse its faithful to more violence, but they also undermined Mr. Abbas's plans and leadership.

We acknowledge that the situation is not simple. The Palestinians asked for 24 hours to act, and the Israelis agreed to wait. Nothing happened. Still, it is far from clear what would have been lost by giving the Palestinians more time. It is true that Mr. Abbas and Mr. Dahlan have been weak-kneed in keeping commitments under the peace plan, known as the road map. But the Israelis have also failed to carry out their commitments on ending settlement activity. Both sides need to act now.

Mr. Sharon must realize that there is no alternative to Mr. Abbas, who is committed to a peaceful two-state solution. If Mr. Abbas is forced from power, it will probably be awhile before anyone else will step forward. That could be the end of the road map — and the road — for quite some time.