William Rees-Mogg / The Times

In 1982, when I had just retired as Editor of The Times, I signed a contract to write a book about the Republican Party of the United States. The book never got written, but I did have one extremely interesting interview, with Richard Nixon in his apartment in New York. I found him intriguing as a man, and highly intelligent.

President Nixon explained to me that a candidate for the presidency had to appeal to his core voters during the primaries; this pushed Republican candidates towards conservative positions and Democrats towards liberal ones. If he won the primaries, the candidate then had to win the national election; he had to move his campaign back towards the centre. Nixon said that this was how he had campaigned when he won the presidency in 1968.

In the past ten days, both the surviving presidential candidates have been making this transition. John McCain had a much easier primary victory than Barack Obama. Hillary Clinton fought the toughest losing campaign since the 1960s. She pushed Barack Obama very hard, and this has attracted more attention to his cautious shift towards the centre. Arianna Huffington's influential website has overflowed with blogposts from ultra-liberals who are afraid they are being betrayed.

In fact, the Obama modifications are by no means betrayals. Most of them do not involve large shifts of policy. What Senator Obama seemed to be saying is that all his policies are subject to development, but he will not do anything stupid, in the light of the facts at the time and the advice he may receive. He is seeking to reassure, not to reinvent.

This is plausible, because it is exactly how he has fought his campaign. In English law, there is the concept of “the reasonable man”. Senator Obama is now campaigning as “the reasonable candidate”. He will adjust his positions to the realities of events, and so he should.

His speeches have not lacked fire and energy, but they are intellectual constructions. This has impressed the college graduate electorate, as compared with the anti-intellectual culture of the Bush White House.

Ten days ago, I went with my wife to California. We stayed at Pebble Beach, the golfing resort on the shores of the Pacific. It was rather a dramatic visit, since the clouds over the hills of California were dark with the smoke of forest fires. California was as dry as tinder and tens of thousands of acres were already blazing.

I do not think anyone visiting the US could fail to recognise the American desire for change. As in Britain there is an incumbent administration that has become progressively more unpopular. The low ratings of George Bush are a handicap to John McCain, just as the low ratings of Gordon Brown are a handicap to the Labour Party. It would now be very difficult for any Republican candidate to win this presidential year.

Senator McCain is a good, independent-minded candidate and would, I think, make a good President. But, after primaries of exceptional pressure, Senator Obama has proved to be an inspirational candidate. He has a very strong appeal to blacks, as one might expect, but also to the young, particularly to the brightest and best of that generation. I cannot remember any candidate with as strong an appeal to this age and ability group since John F.Kennedy nearly 50 years ago.

Americans feel that their nation has been moving in the wrong direction, as many people do in Britain. Yet they have greater confidence than we do in Britain about their economy. The crash of Bear Stearns is taken by most Americans as part of the ordinary wind and weather of Wall Street, when for Britain the run on Northern Rock seemed to be a torpedo amidships. Yet Americans realise that rising prices for food and energy, rising competition from Asia and the global credit crunch will depress their economy in the coming years. If Mr Obama wins, he will inherit big economic problems.

An economic slowdown is usually followed by a change of the party in the White House. Kennedy in 1960, Nixon in 1968, Carter in 1976, Reagan in 1980, and Clinton in 1994 all won with the help of a slowdown. This year the benefit of recession will go to the Democrats.

Some American commentators think that the only events that can now prevent an Obama victory in November would be extreme and unpredictable shocks, what staff college lecturers call “low probability, high impact” events. Of these, the most widely quoted is a possible Israeli attack on Iranian nuclear plants. There certainly exists political pressure inside Israel for such an attack.

However, there are counter-arguments against US support for such an action. American defence chiefs all doubt whether such an attack would be militarily effective; it would play into the hands of Islamic fundamentalists; the political calculation is that it might force McCain to endorse an unpopular war with Iran; it could lead to an interruption of Iranian oil supplies, with the prospect of an oil price rising above $200 a barrel, perhaps much higher.

If the White House does accept these arguments, as I gather it does, there is unlikely to be an Israeli attack in the remaining months of the Bush presidency. Such an attack could hardly go ahead without American approval.

Of course, no one could foretell what attacks might be attempted by al-Qaeda or other terrorists. Osama bin Laden regards terror as a political weapon. If he wants to elect another Republican, he might try one of his “spectaculars” shortly before the election. Such actions cannot be predicted.

The opinion polls are favourable to Obama, but they are too early to be a reliable guide to what happens in November. No doubt the race is still open, but the momentum is on the side of Senator Obama.

I admire McCain, but I think Obama will win.