Middle Eastern issues will likely play a role of unprecedented importance in the American mid-term elections, less than a half-year away. Three topics head the agenda: the course of the Iraq war, the proper response to Iran’s nuclear ambitions, and the soaring price of fuel.

Despite their prominence, these are momentary issues, where voters will make decisions based on transient circumstances and without clearly defined differences between two major parties; what is the Democratic position on Iraq, anyway, or the Republican one on Iran? A fourth Middle Eastern issue, the Arab-Israeli conflict, though less high-profile this year, has deeper electoral significance. It is a perennial topic that helps define the two parties.

The America-Israel bond is the most special “special relationship” in the world today as well as the family relationship of international politics. In many areas – foreign policy, strategic cooperation, economic ties, intellectual connections, religious bonds, and intervention in one another’s domestic politics – the two countries have unusual if not unique relations. This reaches down even to local politics; as a 1994 New Yorker article put it, at times, “it seems that the Middle East – or, at any rate, Israel – is a division” of New York.

In addition, a significant number of Americans (Jews, Evangelicals, Arabs, Muslims, anti-Semites, leftists) vote according to Israel policies.

Since Israel came into existence in 1948, Democrats and Republicans have changed places in their attitudes toward Israel. In the first era, 1948-70, Democrats sympathized more with the Jewish state and Republicans distinctly less so. Whereas Democrats emphasized spiritual bonds, Republicans tended to see Israel as a weak state and as a liability in the Cold War.

The second era began in about 1970 and lasted for 20 years. In the aftermath of Israel’s extraordinary victory in the Six Day War, President Nixon, a Republican, came to see Israel as a military powerhouse and useful ally. This new regard rendered Republicans as positive toward Israel as the Democrats. Noting this reality, I concluded in a 1985 research piece, “Liberals and conservatives support Israel versus the Arabs in similar proportions.”

As the Cold War ended in 1990, the third era began. Democrats cooled to Israel and Republicans further warmed to it. The left made the Palestinian Arab cause a centerpiece of its worldview (think of the Durban conference in 2001), while the right deepened its religious and political alignment with Israel.

This trend has become increasingly evident. In 2000, survey research commissioned by the left-wing, anti-Israel activist James Zogby found “a significant partisan split” on the Arab-Israeli conflict, with Republicans significantly more pro-Israel than Democrats. For example, asked the question, “With regard to the Middle East, how do you feel the next president should relate to the region?” 22% of Republicans and only 7% of Democrats said he should be pro-Israel.

Recent research by the Gallup Poll finds that 72% of Republicans and 47% of Democrats sympathize more with the Israelis than Palestinian Arabs. A detailed look at this same data finds more dramatic results, with conservative Republicans over five times more sympathetic to Israel than liberal Democrats.

The Democratic coolness toward Israel fits into a larger pattern of conspiracy theories about neo-conservatives and anti-Jewish outbursts by such party luminaries as President Carter, Jesse Jackson, Cynthia McKinney, and James Moran. One observer, Sher Zieve, concludes that among Democrats, “anti-Semitism is and has been on the rise” for some time.

The current trend appears to be growing, with an attendant sorting out of Jews and Arabs/Muslims in American politics. This leads me to expect that Muslims, Arabs, and others hostile to Israel will increasingly vote Democratic, even as Jews and those friendly to the Jewish state increasingly vote Republican. In this light, it bears noting that American Muslims see themselves in direct competition with Jews; the Brookings Institute’s Muqtedar Khan predicts that Muslims in America soon “will not only be able to out-vote, but also out-bid the Jewish and most other ethnic lobbies.”

These developments have potentially profound implications for America-Israel relations. The cross-party continuity of policy of the past will end, to be replaced by a major shift whenever the White House changes hands from one party to the other. As the political consensus breaks, Israel will be the loser.

Mr. Pipes (www.DanielPipes.org) is director of the Middle East Forum and author of “Miniatures” (Transaction Publishers).