As President Bush commences his twilight foray into Arab-Israeli diplomacy, he is confronted by a singular and regrettable fact: Israel's long-term survival is not necessarily a given. Threatened by Islamic radicalism, demographic trends, and advances in missile technology, the Jewish state may be living on borrowed time. If he is to help redeem Israel from a tenuous future, Mr. Bush must reiterate one message above all: There will be no peace without a viable Palestinian state.

Bush's nine-day, six-nation tour of the region will be overshadowed by the irony that the outlines of the only realistic solution to the Arab-Israeli conflict have been in place since 1947, the year before Israel was founded. Impelled by the moral imperative of creating a haven for Jews after the Nazi Holocaust, the United Nations proposed that geographical Palestine be divided into Jewish and Arab (Palestinian) states. Sixty years later, a two-state solution remains an indispensable basis for peacemaking.

The obstacles to creating a Palestinian state are formidable. Palestinian attacks on Israeli targets, issues concerning the future status of Jerusalem, water rights, the expansion of Jewish settlements, and the right of return for Palestinians who left or were expelled from Israel in 1948 and 1967 have all bedeviled would-be peacemakers for decades. But if patience with intractability has been an option until now, it no longer remains so, and for three momentous reasons:

The first is the continuing rise of jihadism in the Levant, abetted by Iran and symbolized by the ascendancy of Hamas in Gaza and Hizbullah in Lebanon, both wedded to the goal of destroying Israel.

The second is advances in weapons technology, the implications of which were amply demonstrated when rockets fired by Hizbullah forces in Lebanon paralyzed Israel's major northern city, Haifa, during the 2006 Lebanon war. With extended missile ranges all but inevitable, Tel Aviv itself could one day be at risk.

The third reason, more subtle but more threatening still to Israel, is relentless demographic trends. Jews make up three-fourths of Israel's population but only half that of Palestine in its entirety, which includes the Israeli-occupied West Bank and Gaza. Given higher Arab birthrates, if prospects for a two-state solution collapse, Israel would be faced with the nightmare choice of being a Jewish state or a democracy. It could not remain both. If Israel thus reaches a "South Africa-style struggle for equal voting rights," Israeli Prime Minister Ehud Olmert has bluntly warned, "the state of Israel is finished."

Only once in the 60-year history of the Arab-Israeli conflict has a two-state solution been seriously within reach. Starting in the late 1980s, a confluence of developments simultaneously nudged each of the frontline states to the conflict to the bargaining table. This constellation of auspicious factors, which reached its zenith in the 1991 Madrid peace process and its nadir with the assassination of Israeli Prime Minister Yitzhak Rabin in 1995, has so far proved impossible to recapture.

Palestinian radicals, marginalized in the late 1980s and early 1990s by hopes for peace, now enjoy renewed strength and even exercise political control over Gaza. Coalition politics in Israel gives disproportionate power to intransigent right-wing parties that oppose giving up any land in biblical Israel, including large portions of the West Bank. Meanwhile, Israeli and Palestinian leaders command only limited authority.

The result is that the legitimate national aspirations of Palestinians for a state – and of Israelis for a secure state – have gone unrealized. Frustrated Palestinian nationalism, exploited by Arab governments, has exacerbated a crisis that will only be ameliorated when Israel and the Palestinians come to share Palestine on some secure and equitable basis.

Diminishing prospects for a two-state solution present Bush or his successor with an inescapable crisis, the solution to which will require consummate leadership and risk-taking. Among other things, it will necessitate restoring the US to the position of honest broker in the face of almost certain opposition from a pro-Israel lobby, influential in Congress, that for years has shielded Israel from pressure to make reforms that would have been conducive to the security of the Jewish state.

In the end, resisting such pressures will be critical to furthering the interests of all parties to this conflict. As former Middle East peace envoy George Mitchell once noted, there will never be a Palestinian state until Israel feels completely secure, and Israel will never be completely secure until there is a Palestinian state. This paradox should give both purpose and urgency to Bush's visit.