Citing green issues above all as the challenge being sidestepped, Mr Ashdown launched a blistering attack on the "politics of quietude". He told a rally in London: "In the battle of the spin doctors and sound- bites, the really big issues aren't being debated. That, we are told, is the way it must be. It's the only way to win elections.
"Hasn't Bill Clinton's victory proved it? No tough messages please. Puff small things up into big ones and ignore the really big ones altogether."
In a pointed reference to Mr Blair's drive to appeal to the Tory-voting middle class, he described the lesson Labour seemed to learn from America as: "Don't frighten the horses - above all, don't frighten the middle classes."
He came close to admitting that Mr Blair would win, with another dig at Labour's close links with President Clinton's twice-victorious campaign team. "The politics of quietude may prove - it has proved for President Clinton and Labour hopes it will prove for Tony Blair - a winning strategy for the election. But I cannot think of a worse strategy for a successful government after the election," he said.
He said Britain was "sleepwalking into the next century, and into disasters which are inevitable if we will not face up to what is ahead".
The fierce criticism of Mr Blair's strategy is a significant marker of the distance still remaining between the Liberal Democrats and Labour after a series of recent instances of the two parties working together.
Last week, they set up a joint committee at shadow cabinet level to work on a common programme of constitutional reform, while this week home affairs spokesmen Jack Straw and Alex Carlile jointly presented plans to ban combat knives to the Home Office.
But in his speech to a Real World rally, organised by a coalition of green, anti-poverty and global social justice groups, Mr Ashdown said: "Someone said to me recently: 'The real problem about our politics today is that there are no really big issues to confront us'. Nonsense. Indeed, exactly the opposite is the truth ...
"The tragedy of politics today isn't that the issues are small. The issues are huge. It's the ideas that are small, and because the ideas are small, the politicians are getting smaller too."
He went on: "The deterioration of the global environment is the most serious and difficult practical and real challenge that faces humankind today."
He called for government action to conserve energy, phase out nuclear power, boost renewable sources of energy and charge drivers for polluting city roads. And he repeated his call for taxes to be shifted "away from things we want more of - like jobs - on to the things we want less of, like pollution".
Labour politicians have considered advocating a pollution tax but have rejected it as too risky before an election.Reuse content