Could Jewish voters, normally the most reliable of Democratic constituencies, turn against Barack Obama next year? Until last week, the automatic answer would have been, no way. Then came last Tuesday's special election in New York's Ninth Congressional district, one of the most heavily Jewish in the country and traditionally a Democratic stronghold. This time it went Republican.

There are of course many possible explanations. First, special elections in the US are like British by-elections. When turnout is low, weird things happen. Second, residents of the district, which straddles the New York City boroughs of Queens and Brooklyn, are surely as fed up with Mr Obama's failure to revive the economy as anyone.

The Democratic candidate was not exactly top-drawer either, and some voters must have objected to his support for the law in June that legalised gay marriage in the state. Others rather liked their former Congressman Anthony Weiner who was involved in a lurid sexting scandal. They may have been annoyed with the Democratic leadership for forcing Weiner to resign.

But something else was at work too – Mr Obama's policies towards Israel. A turning point in the campaign was when Ed Koch, the feisty former New York mayor and a Jewish Democrat, endorsed the Republican candidate and accused the president of "throwing Israel under the bus".

In normal times, US Jews, however pro-Democrat, are not single-issue voters; usually the issue of how the man in the Oval Office is dealing with Israel has not been a big factor. For an anxious Israel, however, these are not normal times.

The attack on its embassy in Cairo and the expulsion by Turkey of its ambassador, amid the turmoil of the Arab Spring and its aftermath have left the Jewish state feeling ever more isolated. That sense of isolation will only grow if, as seems likely, the UN General Assembly votes in favour of statehood for the Palestinians this month.

At such moments a country counts its true friends, and no friend has been truer to Israel than the US. It's not that Obama is anti-Israel; he simply hasn't heaped upon Israel the unconditional encomia to which the Jewish state is accustomed from its most important ally – a stance reflecting the need to improve the US's standing in the Arab world.

While we might regard a readiness to criticise Israel as well as the Palestinians as refreshingly even-handed behaviour from an American president, it's highly disconcerting for many American Jews right now. For Obama, facing a re-election battle next year that looks tougher by the week, the implications are alarming.

In purely mathematical terms, the Jewish vote here is not that important. Jews (some 5.5m) may number almost as many as in Israel, but they account for only 2 per cent of the total US population. In political terms, however, they punch well above their weight. Not only are American Jews are a major fundraising force; no other ethnic/religious group votes so regularly – and as a rule 70 or 80 per cent of them vote Democratic.

That predilection ensures that New York, which has more Jewish residents than anywhere on earth bar Israel, is a banker state for Democrats. In Pennsylvania and Florida, two perennial swing states, the Jewish turnout can decide which party wins the White House.

In Florida, Jewish voters, disproportionately elderly retirees, account for 8 per cent of the turnout; the failure of some of them to understand the "butterfly" ballots used in heavily Jewish Palm Beach may have cost Al Gore victory in 2000. In Pennsylvania, Jews account for 5 per cent of the vote; their disaffection could be disastrous for Obama in a state expected to be very close-fought in 2012.

Not surprisingly, Republicans sense an opening. For years, the changing nature of the party – above all, the rise of the Christian conservatives – has been strengthening the embrace of Israel. "God's Foreign Policy" is how support of the Jewish state is described by some white evangelicals. Michele Bachmann, a White House candidate, even declared that "Nations receive blessings as they bless Israel".

Not by coincidence, with the vote at the UN approaching, the The Wall Street Journal and The Jerusalem Post carried an op-ed article on Friday by Texas governor Rick Perry, a front- runner in the Republican race, lambasting the president's policy on Israel and accusing the Palestinians of undermining hopes of peace by their reckless statehood gambit.

History, too, must add to Mr Obama's unease: getting tough on Israel is bad for re-election in the US. By the 1980 election, Jimmy Carter was perceived as overcritical of the Jewish state. He won a bare 45 per cent of the Jewish vote and lost to Ronald Reagan by a landslide.

A dozen years later, George Bush senior tried to bring Israel to heel by blocking US loan guarantees for building new settlements, and was beaten by Bill Clinton. The loan guarantee row may have had little to do with his defeat, but the historical pattern escapes no one: Mr Obama lost a comparable settlements showdown with Israel. Polls suggest that his support among Jewish voters has dropped from 80 per cent in 2008 to a near Carter-like 55 per cent.

Small wonder the White House has appointed an "outreach" director to improve relations with the Jewish community, and dispel such myths as the President's demanding a return to the 1967 borders as part of a final peace settlement. Foreign policy normally plays little part in a presidential election. Small wonder, too, that Israel next year could be the exception.