Tony Blair flies to Washington today in a bid to prepare the ground for a successful Group of Eight summit at Gleneagles. He has a tough task ahead. With only a month to go and public pressure mounting, the summit is poised between triumph and disaster.

Mr Blair and Gordon Brown, the chancellor of the exchequer, have raised expectations with talk of a summit that will go down in history for tackling climate change and poverty in Africa. The promise of a Live 8 concert to put pressure on the G8 promises to raise public excitement still higher.

Mr Blair and Mr Brown have embraced Live 8. They see its potential to mobilise support. But there are dangers. Planned marches could easily turn into anti-globalisation demonstrations. Irresponsible talk of 1m campaigners descending on Edinburgh raises fears of disruption from the anarchists who wrecked past summits.

The G8 may have to chose between winning the acclaim of the activists and making serious inroads into deeply intractable problems that cannot be solved in three days at a Scottish golf course. Worryingly, the building blocks are still not in place. Europe remains locked in a ridiculous dance over rival pet projects to finance extra aid. But the big differences - and they are very big - are between Europe and the US.

Mr Blair must persuade Mr Bush to engage. This means setting goals that are realistic in a US context. But it also means setting limits on how far he will go to win US backing. The US will not commit to United Nations aid targets or join Mr Brown's overhyped International Finance Facility. It will not agree to Kyoto-style emissions targets, either. But there is scope to make real progress on both fronts.

Mr Blair should push Mr Bush to sign up to a doubling of G8 aid to sub-Saharan Africa by 2010. The EU will meet much of this increase. Debt relief will help release funds, but only $1bn-$2bn of the additional $25bn (£14bn) that Mr Blair's Africa commission says is needed. The prime minister must not give in to populist demands for World Bank write-offs unless Mr Bush agrees to replenish the Bank's funds.

There is more to helping Africa than aid. Mr Blair should ask Mr Bush to help countries attain good governance, not simply reward those that achieve it, and seek backing for a bold G8 offer to end all agricultural export subsidies and spur on the Doha trade talks.

On climate change, Mr Blair must not settle for talk of technology and technology transfer alone. Technology matters, but there has to be demand for it. As an interim step, it would be good to win an unequivocal statement on the science of global warming, and to unblock debate on life after Kyoto, perhaps through a working party of G8 and big developing countries. What Mr Blair cannot do is accept an agreement without a statement on the science: succouring those who claim that the science is still disputed would be worse than no climate deal at all.