A Gallup poll released last week found that President Barack Obama’s approval rating stood at 60 percent among Jewish Americans—an 18 point drop from the 78 percent of Jews who voted for him in 2008.“The question,” suggested the Jewish Telegraphic Agency’s Ron Kampeas “is whether Obama’s Jewish popularity dip since ’08 stems from the same cause of his fall generally—America’s persistent economic problems—or whether it has to do with the president’s policies on Israel.” He continued: “Apparently the interpretation depends on who is answering. Democrats and Gallup say it’s the economy; Republicans say it’s Israel.”Yet Kampeas’ either-or reasoning may have missed a more fundamental and intriguing point: Jews almost always turn against a sitting president.If Barack Obama flew to Israel to offer Benjamin Netanyahu a back rub, he would lose Jewish voters. If he raised, lowered, or kept taxes the same, he would lose Jewish voters. If he learned Yiddish or put a menorah on the flag, he would lose Jewish voters.For more than 30 years, every incumbent president running for re-election, with a single exception, has lost Jewish supporters in his second campaign.Running against the unelected incumbent Gerald Ford in 1976, Jimmy Carter won 50 percent of the popular vote and 71 percent of the Jewish vote. Four years later, when he was defeated in his re-election campaign by Ronald Reagan, Carter won 41 percent of the popular vote, a decrease of 9 percentage points, and only 45 percent of the Jewish vote, a remarkable drop of 26 percent points.In 1984, a wildly popular Reagan won re-election with 59 percent of the popular vote, an 8-point increase over his 1980 share. But Reagan’s percentage of the Jewish vote decreased by 8 percentage points from his first election to the second—which meant he underperformed among Jews relative to the general voting population by 16 points in his re-election effort, just as Carter had in his unsuccessful try four years earlier.George H.W. Bush fared 24 percentage points worse among Jewish voters in his unsuccessful 1992 re-election effort, while Bill Clinton’s take of the Jewish vote dropped by only 2 percentage points from 1992 to 1996. But because Bush’s performance among the general population also decreased from his first election to his second while Clinton’s improved, both men underperformed by 8 percentage points among Jewish voters relative to the general electorate.All that said, the majority of Jewish voters are loyal. Even in a very bad year, a Democratic presidential candidate can be assured of at least 45 percent of the Jewish vote. Another 14 percent of Jews—like those who voted for third-party candidates like John Anderson in 1980, who got 14 percent of the Jewish vote, or Ross Perot in 1992, who polled 9 percent of Jews—might not vote for a Democrat but still won’t vote for a Republican. There’s 10 percent of American Jews who will reliably vote for a Republican presidential candidate, plus 9 percent who may vote independent but won’t vote for a Democrat, based on this historical data.That leaves about 22 percent of Jewish voters up for grabs, most of whom at least lean Democratic. In 2008, Obama won 82 percent of these toss-up Jews. This gave him 78 percent of the Jewish electorate compared to the 53 percent of the general electorate.The most obvious conclusion to be drawn from all this—that Obama will inevitably lose Jewish voters in 2012—will make a fine talking point for Jewish Republicans. But Democrats of a more optimistic bent might look to recent history for hope. In 2004, George W. Bush bucked the trend and increased his initially scant Jewish vote by 5 percentage points, compared to a popular-vote increase of 3 percentage points. (Republicans would likely argue that Bush’s increase in Jewish support was hardly an accident, but rather the clear product of his stance as Israel’s leading supporter and an unshakable friend of its former prime minister, Ariel Sharon.) Perhaps that will become the new trend; after all, from Franklin D. Roosevelt’s first re-election through Richard Nixon’s resignation, Jewish support for incumbents increased as predictably as it decreased from Watergate to September 11.