It is hard to see who can stop Senator Barack Obama becoming the next President of the United States. He has built up an excitement such as no candidate has created since President Kennedy in 1960. He is, in my view, a better speaker than Kennedy. Like Kennedy, he combines personal magnetism with a strong appeal to American idealism.

Like Kennedy, he is young and speaks for the new generation of American politics. By ordinary political reckoning, 2008 ought to be the Democrats' year. In 2006 they captured both houses of Congress in mid-term elections.

There are, of course, hypothetical events that could change everything. There could be an attack on Mr Obama himself, but he is protected by the Secret Service. There could be an action by al-Qaeda, which would refocus American anxiety on the threat of terror.

But al-Qaeda is itself highly political. It would probably not be in its interest to secure the election of Senator John McCain. Al-Qaeda may be unpredictable, but it would be a mistake for it to interfere in American politics, even if it had the capacity to do so.

At the start of the primaries, when all eyes were on Iowa and New Hampshire, Senator Clinton was the frontrunner for the Democratic Party nomination. She had the organisation, she had the money, she had the name recognition, she had the professionalism; she even had Bill Clinton, even if he is something of an unguided missile.

But those days are now long ago. Senator Clinton has fallen behind Senator Obama in almost all of these factors, except for Bill Clinton's support. Senator Obama has captured the public's imagination, and gone ahead in the polls, but he also has more money, a better organisation and valuable endorsements from all sectors of the Democratic spectrum. He is now ahead in delegates.

Theoretically, Hillary Clinton could still finesse the nomination, possibly by holding on to the “super delegates” (senior party members appointed to the Convention), though they are free to switch to Obama whenever they wish; some have already done so. She could also try to instal the Florida and Michigan delegates, though their primaries were invalidated because their states tried to steal a march by holding primaries early. These were primaries without campaigns.

However, the Clintons are already suspected of clever tricks, whether fairly or not.

If Senator Clinton takes the nomination away from the presumptive “first black President of the United States” by playing games with the delegates, she will alienate the electorate. Her image would be that of the Wicked Witch of the West - she would become unelectable.

One has to remember that there is already a deep undertow of Clintonphobia among American voters, not only among Republicans. A divided Democratic Convention, with Hillary emerging as the nominee as a result of challenged votes from Florida or from super delegates, would virtually guarantee a Republican victory.

This means that the Democrats will have to ensure that the candidate who is nominated is the one who gets the most delegates from the primaries and the caucuses. The super delegates cannot afford to use their power to override the popular vote.

Senator McCain is another matter. I am a McCain admirer; if I were an American, I would almost certainly vote for him. He has the key qualities a President needs - courage, intelligence, humanity, independence, experience of international affairs and sufficient self confidence to support the most intolerable role in the world. As a prisoner of war in Vietnam he was tortured and his behaviour was heroic. If one regards the security of the world as the supreme concern of the President of the United States, one would want John McCain to be the next President. He is probably a wiser man who knows more about war than any President since Eisenhower in the 1950s.

Senator McCain has an advantage over Senator Obama; he is already assured of the Republican nomination, when Obama is still under fire from Hillary Clinton. She may do some damage. Naturally, Senator Obama has briefer and in some ways narrower experience than McCain. The President has to be Commander-in-Chief; undoubtedly Senator McCain is far better qualified for that aspect of the President's function. Anything that emphasises global threats to the United States would focus attention on Senator McCain's strengths.

Yet the core argument of the Obama campaign is both powerful and timely. In American politics each generation looks for a renewal. That may come from either party; it is not simply a matter of a swing from the right to the left. In the first half of the past century it came from Theodore Roosevelt as a Republican and from Franklin Roosevelt as a Democrat.

John F. Kennedy offered renewal in the 1960 election, though an older man, Lyndon Johnson, passed the legislation that gave reality to Kennedy's promises. There was a renewal of hope in the Ronald Reagan presidency but there has been little renewal since Reagan's time, which ended 24 years ago.

Senator Obama offers a new generation of ideas that appeal to a new generation of voters. He is the presidential candidate of the young. Senator Clinton and Senator McCain belong to older generations. Americans do not want to return to the issues of the 1990s with Clinton, let alone the 1960s with McCain. Senator Obama identifies with the issues of the 21st century. In 2008 these issues are more relevant than those of a generation ago.

Politicians who offer hope win elections. Despite his age, John McCain offers an alternative Republican programme. He is not a neo-Conservative and would be very unlike George Bush. Hillary Clinton would represent only too clearly a third Clinton term. Barack Obama has the future of America ahead of him.

Times Recommends

• Strip away the figleaf and reveal naysayers
• Dave’n’George: there may be trouble ahead
• I do not accept that £64,000 a year for politicians is peanuts