So this is how the Clintons end – not with a bang but with a whimper? By this time next week Barack Obama will be the Democratic nominee. Yes, I’ve thrown a little salt over my shoulder, crossed myself a few times and said 10 decades of the rosary, but the laws of mathematics have to be worth something.

There remains the unknown quantity of the Clintons’ miraculous ability to produce high drama from delegate calculus – “You can’t tell how far a frog will jump until you punch him,” Hillary Clinton mystifyingly warned last week – but the overwhelming probability is that by the end of the week she will have to decide either to fight on, pointlessly, until the Democratic convention in August or make a nice concession speech and wait a few months, if Obama loses the presidency, before saying “told you so”.

By Wednesday all the votes from the primaries in Montana, South Dakota and Puerto Rico will have been counted and many of the disputed Michigan and Florida delegates ceded to Clinton, but still she won’t have enough to win. Obama will still need the support of the super-delegates – party officials who have a vote at the convention – to make him the undisputed winner. But there are many signs that the numbers he needs will begin to declare themselves in a wave after the final primary votes come in.

We still don’t know fully how the Clintons intend to respond to this extremely inconvenient truth. So far as I can tell, very few people have yet approached Her Majesty to ask what she actually intends to do. If you want to have a good idea of how dysfunctional and Bush-like a Clinton administration would be, that’s a good place to start: a leader whose closest aides are often afraid to talk to her.

Her departure – drama-laden or not – will transform the electoral scene. American presidential elections are somewhat different from British elections because they focus on two people alone. And with Obama and John McCain it would be very hard to imagine a more arresting or fascinating contrast.

You have in one corner an iconic figure of the late 20th century: a naval man, famous prisoner of war, torture victim and youthful lothario and trouble-maker. McCain is a product of the west, Arizona and the military. And yet he is also extremely comfortable in elite circles, loves gabbing with journalists, enjoys the company of Hollywood moguls and stars and feels uncomfortable with religious fanatics.

Despised by many hardcore conservatives, he would be the oldest first-term president and closer to the Democrats than the Republicans on issues such as climate change, campaign finance and immigration. Alternately charming and volcanic, conciliatory and dogmatic, he remains an enigma as a potential president, a job that he wanted (and deserved) eight years ago.

Against him we see a walking symbol of 21st-century America. A mixed-race son of a divorced mother, reared in Indonesia and Hawaii, Obama is an American whose father was a member of the Kenyan elite and whose great-uncle liberated Buchen-wald concentration camp in 1945 as part of the US army. He is the first unashamed liberal to run for president since George McGovern, but a man whose capacity for reason and inclusion and civility has won him many conservative friends and admirers.

A bi-racial man who does not condone but will equally not disown the angrier segments of black America, Obama glides through public life like a visible incarnation of Ralph Ellison’s Invisible Man. He is as cool as McCain is hot.

As the Clintons fade ungraciously away, the emergence of these two from the dust of an astonishingly vivid and endless primary campaign comes to me, at least, as a massive relief. These two men are easily the best each party has to offer, the two most capable of talking to the other side: serious, decent, principled figures with, of course, their fair share of political shading. And in a war against Islamist terror, which for me remains the most important issue, they offer a choice as stark as it is difficult.

The biggest worry about Obama is whether he will be too reflexively diplomatic. Does he believe that some of America’s enemies are reasonable in a good way rather than rational in a malign way? How will he respond if our enemies attack? His defenders point to his diffident but tough composure in this campaign as a sign of his steeliness. He has supported military strikes in Pakistan, they say. He was trained in urban Chicago politics, they remind. We’ll see.

A more telling question for me will be how he adjusts to new realities and possibilities in Iraq. Recent successes for the Maliki government and Iraqi army in Basra and Sadr City and the lowest level of civilian deaths in four years suggest that Iraq has altered for the better. Can Obama adjust his strategy – so we withdraw in the best way possible for our interests?

With McCain there’s a reverse worry. Has he become more neocon than Bush? In the past McCain has been known as a pragmatist and realist, able to see when American interests have to come before American rhetoric or sentiment. But in the past few years, as the Iraq debate has polarised so many, he has become shriller and more demagogic on the war in the Middle East, more prone to Bush-style declarations about good and evil than subtler assessments of how best to mix force, diplomacy and multi-lateralism to the West’s advantage.

And so one worries: has his admirable sense of the danger of our foes blinded him to ways in which a defter diplomacy and shrewder deployment of force can help to advance our inter-ests? Does he understand the need to appeal beyond Muslim leaders to Muslim populations? Is he temperamentally suited to the delicate chess game of the new global politics?

Let’s say both broad worries about both men are salient. The question then becomes: is Obama more capable of adjusting to toughness or is McCain more capable of adjusting to nuance? Neither is perfect. The job that Americans will soon face is to figure out who is more perfectable in office. My sense at this point is that Obama is more capable of strength than McCain is of subtlety. And that McCain’s domestic weakness with his own base may force him into cruder measures than are appropriate to the threat we face.

But these are, perforce, preliminary judgments. We need a longer, deeper campaign between these two to see where the contours of the next presidency will fall. And for that to happen the Clintons still have to leave the building. They have to grasp at this stage in their lives that it isn’t always, everywhere, about them.

We’re waiting. Until the last frog is punched.

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