Unfortunately the cold war was declared restarted while he was away. McCain's ideas about what to do vis-a-vis Russia and Georgia are insane and would likely start a third World War, but he sounded ''tough'' and ''experienced'', and alas that's all that matters. Obama was in trouble. The story of McCain's seven houses and the generally well-received selection of Joe Biden as his running mate put a tourniquet on the bleeding and gave Obama a bit of momentum going into the con-vention. So what must he do with it?

I see four major tasks at hand. First, Obama needs to find a way to talk about the economy and people's hardships that's clear and direct. This language has eluded him so far. He's an intellectual. His heart isn't in bumper-sticker talk. He sees nuance. This is a good thing, for governing. But for campaigning it's a decidedly bad thing. He needs to find a phrase (Bill Clinton had ''putting people first'') that expresses a populist economic outlook and hammer it home.

Second, he needs to put some emphasis on the theme that made him famous in the first place, back at the 2004 convention, the idea of ''post-partisanship'' and one America. There is not a red or a blue America, he said then; there is a United States of America. Many liberal activists pooh-pooh this notion as pretty talk that signals weakness to evil conservatives, but they're wrong. Average Americans like this idea. Nearly a third of the electorate don't belong to either party and are attached to no particular set of ideological beliefs. They will decide the election. They'll absorb some tub-thumping and even a little dose of class warfare on economics, but they want a leader who will make an effort to move us beyond our red-blue civil war. Third, he needs to give the Clintons their day in the sun and let Hillary's supporters vent whatever it is they need to vent. About 20 per cent of her voters still say they're planning to vote for McCain. This is childish and ignorant beyond belief, but it is what it is. In a more just world, Obama would be able to stand at that podium and say: ''Are you people nuts? You're going to vote for a man who'll appoint supreme court judges who'll outlaw abortion rights because you're mad at me and Howard Dean?'' I wish he would, but of course he can't. He barely beat her, so he is in the position of having to smile as the Clintons act out their dramas. Whittling that 20 per cent down to 5 per cent won't happen this week. It'll happen in October. If it happens.

Fourth, and most important, this convention needs to establish a clear negative narrative about McCain. I've been saying this since May but I'll keep saying it until it happens. The Obama team simply must find effective ways to attack McCain; not his positions but his character. We all admire his suffering in Vietnam, but that does not mean the rest of his life and career are without blemish. It would be nice if presidential campaigns were about who has the better healthcare policy, but they aren't. They usually end up being about which side has launched the more effective attacks. At the 2004 convention, the Democrats made the foolish decision not to allow any attacks on George Bush. He was a ''wartime president'' and Democrats thought they'd be high-minded about that. They paid the price. There's no teacher awarding gold stars. There are only voters. They may say they don't like negative campaigning, but they always respond to it. Don't let them down.

Michael Tomasky is editor of Guardian America.