Turning a Congressional special election into a referendum on President Obama’s policies towards Israel makes perfect sense – to former Mayor Edward Koch. Koch, after all, thought up the idea and is off and running in pursuit of his plan to treat the local contest for a Queens-Brooklyn seat like an exercise in foreign policy.

To pretty much everyone else, even friends and supporters who generally agree with him, Koch’s latest quest is quite a stretch in both logic and politics.

The former mayor just officially announced that he is supporting Bob Turner, a retired Republican businessman, in the Sept. 13 election to fill the seat vacated by disgraced Democrat Anthony Weiner. Turner shares Koch’s objections to President Obama’s policies towards Israel, and promised the former mayor that he would fight any Republican attack on Medicare, Medicaid and Social Security.

There is a bit of a complication in Koch’s strategy, though. Turner’s Democratic opponent, Assemblyman David Weprin, is as critical of Obama’s Israeli policies as Turner is. Weprin is also an orthodox Jew with family in Israel, and a Democrat in the Brooklyn-Queens district where registered Democrats outnumber registered Republicans by 144,000. The area has been represented by a Democrat in Congress since 1924; in 2008 the district gave Obama 55 percent of the vote.

Obama’s subsequent policy statements about the Middle East, particularly his call on Israel to return to its pre-1967 borders (as did President George W. Bush, though in milder language) irritated many Jewish voters. For Koch, it was the proverbial last straw, convincing him that the president is not trustworthy on Israel. Obama also has plenty of allies among Jewish voters, who are hardly monolithic.

For Koch to prevail, those 9th district Democrats who share his criticism of Obama have to be willing to cast a vote for Turner and against Weprin for purely symbolic reasons. They have to break with their party affiliation and say to themselves, ‘O.K., I know Weprin shares my position on Israel and that he supports the continuation of Medicare, Medicaid and Social Security. But I am going to vote for Turner to send a message to Obama.’

Many Democrats will vote for symbolic reasons, predicts Koch, who has broken with Democrats before over Middle Eastern politics, most memorably in 1980, when he sharply distanced himself from Jimmy Carter over Israel during the incumbent president’s race ran against Ronald Reagan. A Turner victory would “send a message that the Jewish community is no longer in the pocket of the Democratic party,’’ says Koch.

Weprin strategists, who question Turner’s commitment to the Big Three entitlements, say they doubt Koch’s approach will prevail.

Not that it matters much who wins. Ordinarily, the victor on Sept. 13th would fill out Weiner’s term and face another election in a year. But the 9th district is likely to disappear by then, through redistricting. The state has to lose one Congressional district upstate and one downstate because of a decrease in population and the 9th has been targeted.

Why, then, bother with all this? Because it is not about the Weiner district, really. It is about the next presidential election: As Koch sees it, if a Democratic district with a large Jewish population elects a Republican to Congress, Obama and national Democrats will have to take notice. “They will have to think about 2012,’’ reasons the former mayor.

There you have it. The 9th Congressional district of Queens and Brooklyn as arbiter of national and geopolitics. Or not.