Rupert Cornwell: Today, Jerusalem. Tomorrow, Washington?

Life offers few greater pleasures than catching up with old friends. Take Mitt Romney and Benjamin Netanyahu. The pair were in their late twenties, rising stars at the pioneering management company, the Boston Consulting Group when their paths first crossed in the mid-1970s. Back then, Bain Capital was no more than a distant gleam in Romney's eye, while Netanyahu was in the US to hone his business skills after fighting in an Israeli special forces unit during the Yom Kippur War.

But they've kept in touch, and now one is prime minister of Israel, while the other might soon be president of the United States. One way and another, there's a lot to talk about. But that's only one reason why Romney is off to Israel next weekend, after dropping in on the London Olympics.

Another consideration is the need to prove he knows something about foreign affairs (not Romney's strongest suit). Above all, though, he is fulfilling what has become an obligatory rite of passage for aspiring US presidents: a public appearance on Israeli soil, to display undying solidarity with the Jewish state.

Oddly, once they are elected and forced to deal with the Middle East's intractable realities, US presidents are in much less of a hurry to visit America's closest regional ally. Ronald Reagan never went to Israel, nor did George HW Bush. Bush Jr, perhaps America's most pro-Israel president yet, waited until his final year in the White House to do so; Barack Obama has yet to go as president.

But when you're a candidate, it's different. Romney has already visited Israel twice. Sarah Palin went in 2011, when everyone assumed she would be running this year. And at this very moment in the 2008 cycle, having finally seen off Hillary Clinton, Democratic Senator Barack Obama went to Jerusalem to proclaim his "unshakeable commitment to Israel's security". Such statements of course are not meant to bring about the miracle of peace with the Palestinians, or to instantly counter the Iranian threat. They are solely about winning votes back home. And you don't do that by upsetting America's mighty pro-Israel lobby.

In absolute terms the Jewish vote here is small, 2 per cent or so of the electorate, and for many of these voters, other issues are at least as important as the fate of Israel. But their clout is out of all proportion to their numbers. And this is not merely because some of the largest Jewish-American populations are in closely fought swing states such as Florida.

Some of Israel's most vociferous supporters are conservatives and evangelical Christians, a major segment of the electorate. Offend them by sounding lukewarm about Israel and you are in serious trouble. And then there's money. Jewish groups have long been major campaign donors, but in the age of the Super Pac and unlimited personal donations, their importance has only increased. Newt Gingrich's 2012 White House bid, for instance, was bankrolled by the Jewish casino tycoon Sheldon Adelson, for whom support for Israel is the most important issue (and which may explain why Gingrich breezily dismissed the Palestinians as an "invented people".)

And so to what many see as the spider at the centre of the web, the Lobby – in other words Aipac, the American Israel Public Affairs Committee. Of the many Jewish groups, Aipac is the most influential. No fewer than 13,000 delegates attended its 2012 annual conference in Washington, addressed by President Obama and every Republican presidential candidate, outbidding each other in declarations of fealty to Israel. Its influence on US Middle East policy is legendary; if America's inbuilt bias towards Israel is the biggest obstacle to a Palestinian settlement, as many contend, then Aipac is probably the biggest reason why.

Is it really that powerful? Incur the wrath of the Israel lobby, it is said, and your political career may be doomed. Exhibit A for this theory is Republican Senator Charles Percy, who was defeated in 1984 after he had crossed Aipac by supporting the sale of US military aircraft to Saudi Arabia, a step vigorously opposed by Israel. Whether or not that is true is beside the point. Power lies in the perception of power, and Aipac is perceived to be very powerful indeed. Just how powerful, even the Obama campaign may now be starting to wonder. In every election, Jewish-Americans are a reliable Democratic constituency; four years ago they went 78 per cent for Obama. But this time Republicans sense an opening, as they assail the President for his "disdain" for Israel – manifest in his demands for a halt to new settlements, his use of the word "occupation" for the Palestinian territories, and his clear opposition to a military strike against Iran's nuclear sites.

Obama will surely carry the Jewish vote again, easily. But polls suggesting his approval rating among Jews has slipped, and the Democrats' loss in 2011 of a heavily Jewish New York district in a special congressional election, have given the Republicans hope. All this may be wishful thinking. But it will be one more topic for two old friends to chew over in Israel next weekend. And one thing may be safely predicted. If Mitt Romney wins, and Benjamin Netanyahu remains prime minister, US ties with Israel will never have been closer.