Liberal Democrat leader Sir Menzies Campbell has accused Tony Blair of "squeezing the values out of British politics" during his 10 years in power.
Sir Menzies said Mr Blair would leave with much disappointment
He told the BBC there was "a real opportunity for a party of principle like ours" in Thursday's elections in England, Scotland and Wales.
Sir Menzies, visiting Watford, in Hertfordshire, said the Iraq war would follow Mr Blair "into retirement".
He also criticised rises in interest and inflation rates and violent crime.
Sir Menzies said: "I would give him (Mr Blair) four out of 10 for the performance of this government, considering all the hope there was. In the last five years they have been a disappointment."
He added: "The prime minister won three general elections (but)...he will go, like Mrs Thatcher, with a great deal of disappointment surrounding him.
"He has squeezed the values out of British politics. That's why there's a real opportunity for a party of principle like ours....
"He's left a great deal for his successor to do."
Sir Menzies called the Iraq war "the most damaging foreign policy decision since Suez in 1956. Just as Suez followed Anthony Eden into retirement, so Iraq will follow Tony Blair".
On Mr Blair as prime minister, he said: "My main memory is of the day he began. People of all parties and none thought things could really get better. In the early years there was some promise of that."
He added: "Somehow, for reasons I've never been able to understand, he became very abashed and drew back from root-and-branch reform."
Sir Menzies also criticised Chancellor Gordon Brown, seen as the prime minister's likely successor.
He said: "We have no idea what he will do when he gets into Number 10....
"People are waiting with some apprehension to see whether he will be a genuinely reforming prime minister or whether what he does will be subject to the same media manipulation as the Blair government."
In a speech earlier, Mr Blair said 10 years of Labour government had left the UK stronger, fairer and better.
And writing in The Sun newspaper, Mr Brown said he was "honoured" to call Mr Blair his "oldest friend in politics", while admitting there had been "ups and downs" along the way.