This presidential battle is only just starting - and it is going to get so much nastier

Until the Republicans have held their convention, there will be no clarity about how Mr Bush plans to marshall his defences

How desperately, desperately, America's Democrats want to believe. They want to believe that they have a credible presidential candidate. They want to believe that the force of argument - their arguments - will prevail. They want to believe that they can garner enough votes in November to remove George Bush from the White House. And it is not only America's Democrats who want to believe, but a sizeable slice of European opinion as well: we want to believe it, too.

How desperately, desperately, America's Democrats want to believe. They want to believe that they have a credible presidential candidate. They want to believe that the force of argument - their arguments - will prevail. They want to believe that they can garner enough votes in November to remove George Bush from the White House. And it is not only America's Democrats who want to believe, but a sizeable slice of European opinion as well: we want to believe it, too.

The fervour with which so many people on both sides of the Atlantic are rooting for change - fervour of an order quite different from that of either main party in recent presidential campaigns - has been evident since the earliest Democratic primaries and was on full display this week in Boston. But it is also the reason why extreme caution is in order when we contemplate John Kerry's achievement at the Democratic Convention. To borrow from one of the rhetorically more successful passages of Mr Kerry's acceptance speech: "Proclaiming 'mission accomplished' doesn't make it so."

The Democrats were forgiving of Mr Kerry in Boston, and they were a willingly malleable audience. Softened up by Al - "every vote counts" - Gore, recalling how he was cheated of the presidency despite winning the popular vote, then by Bill Clinton - rekindling, well, just the glory days of Bill Clinton at the top of his political game - they were primed to give their candidate every benefit of every doubt. Mr Kerry's wife, Teresa, avoided mis-speaking in expressing her loyalty and admiration, Mr Kerry's running-mate, John Edwards, modulated his youthful brilliance, leaving ample room for the lead candidate to shine, and Mr Kerry did the rest.

Long senatorial experience clearly does not automatically confer assurance with the hi-tech autocue and he could learn some lessons from Mr Edwards on varying speed, volume and tone to get his message across. All in all, though, he evinced competence and an appetite for the job, which is about as much as most of his audience had realistically hoped for and rather more than some had feared.

He was not hesitant; he was not, at least not excruciatingly, dull. He had some felicitous turns of phrase, including a quip about not wanting to claim that "God is on our side", but rather "to pray, humbly, that we are on God's side". And his introduction - "I'm John Kerry and I'm reporting for duty" - lent a touch of real genius, and perhaps supplied a leitmotif for the long campaign to come.

For the rest of his 50 or so minutes, Mr Kerry pressed all the necessary buttons, one after the other, firing missile after missile in Mr Bush's direction, trying to answer every question ever asked of him, anticipate every counter-attack. Would John Kerry make a brave and reliable war leader (despite his anti-war campaigning when he returned from Vietnam)? Absolutely; here were his former comrades-in-arms to prove it. Was his upbringing as a diplomat's son mostly abroad a liability? Absolutely not: he had learnt what it meant to live in the freedom zone of a divided world. Was he inconsistent, did he know his own mind, was he guilty - in American parlance - of political "flip-flopping"? Not a bit of it: he was proud to see complexities "because some issues just aren't that simple": take those non-existent weapons of mass destruction, that war fought "on the cheap". Was his patriotism open to question because he insisted that America could do better in forging alliances abroad and enacting social justice at home? Perish the thought: "That flag doesn't belong to any president, ideology or political party; it belongs to the American people."

Most ambitiously, he tried to neutralise some of Mr Bush's appeal to the Heartland, implicitly attacking his overt religiosity, his cultivated war leader's image, his invoking of family values that did not necessarily "value families" and - with reference to US dependency on imported oil - he called for "an America that relies on its own ingenuity and innovation, not the Saudi royal family". It was a low blow, but one that drew some of the loudest cheers of the night.

It was not an elegant speech, either in conception or delivery, but it had more than a germ of effectiveness. One measure of this was the speed with which his rival set out for the swing state of Missouri, affected a folksy faux-Texan accent and half-joked that one of the most compelling reasons to re-elect him was to give his wife Laura - whose ratings soar above his - another four years as first lady. Laura, he did not have to say, is a down-home wifely companion, nothing like that sophisticated, independently moneyed, double-barrelled Teresa Heinz Kerry, who tells uncooperative journalists to "Shove it". Scratch.

We can expect more, much more, in a similar and doubtless nastier vein, as the campaign progresses, and the battle is far from fully joined. Until the Republicans have held their convention, against the emotive anniversary backdrop of 11 September New York, there will be no clarity even about how Mr Bush plans to marshall his defences. Over the next month, any impression of Democratic ascendancy will be quite false, however much praise is lavished upon Mr Kerry's performance.

This year's is an unusual presidential election, not only because the Democrats are so fiercely united in their purpose, but also because national security and foreign and defence policy will be central to the campaign. Among the first questions voters will be asking are who will make the more decisive war-time commander-in-chief, who is the truer patriot, who will keep America safer? As the Kerry team well understands, these are perilous questions for a Democratic contender, especially one who was a war protester as well as war hero.

The last time national security figured anything like as prominently in a presidential campaign was probably 1984, when Ronald Reagan worsted Walter Mondale by conjuring up the vivid spectre of Cold War danger: "There's a bear in the woods." Mr Kerry makes a more credible bear-hunter by far than Mr Mondale. But it is the incumbent leader, not the challenger, who enjoys the prerogative of summoning the bear as needed, then claiming the credit for slaying it. What no one can forecast is whether Mr Bush's White House has warned of the terrorist threat once too often since 11 September 2001. If he invokes mortal danger one more time, will he come across as the intrepid warrior, the trusty protector of the nation, or as a scare-monger, armed to the hilt, who fears fear itself?

As his Democratic challenger told us on Thursday night, he is John Kerry and he is reporting for duty. This week he passed his fitness test. But we still do not know whether he will graduate to commander-in-chief.

m.dejevsky@independent.co.uk