Shadow foreign secretary William Hague today warned party activists not to be complacent about winning the next general election.
He said Labour would run a "wholly negative" campaign characterised by smears and playing on people's fears.
Mr Hague told the Conservative Party conference that if Gordon Brown won the election "the last traces of hope and optimism and confidence about our national future would drain away".
Despite the opinion poll leads and local election successes, senior Conservatives have been careful to avoid sounding as if the general election was already won.
Mr Hague, effectively the Tories' deputy leader, said: "Whatever our successes and however much the country cries out for change, we must never allow one morsel of complacency to creep in to our campaign.
"We must be conscious that the system is stacked against us - that Labour only have to draw to win a majority in the House of Commons but we have to win by some two million votes to do the same.
"We must be conscious too that this election, as we saw in Brighton last week, will bring forth from Gordon Brown's Labour a wholly negative campaign - barricaded in the Downing Street bunker they will fling any dirt, stoke any fear, spread any smear and peddle any distortion to scare people into thinking that change is dangerous, honesty frightening, and the fresh air of new leadership actually poisonous for the people of Britain.
"This campaign will have ups and downs, it will have moments of difficulty, when we will need to keep our nerve, calm our friends, and make sure that we always march in step towards the goal of a better government for our country."
After the Prime Minister's conference speech last week, in which he reeled off a list of Labour's achievements, Mr Hague gave his own version detailing the Government's failures on public finances, crime and health.
He mocked Mr Brown over the "goats" that joined - and subsequently left - the Government of All the Talents.
"The arrival of a string of ministers from outside politics - Lord Malloch-Brown, Lord Darzi, Lord Digby Jones, was hailed by Gordon Brown as showing it was a Government of All the Talents.
"It turned out they were so talented that after working with him for a short time they left, and so we now have a Government of All the Talents with all the talents taken out of it."
Mr Brown and his government "do not possess the quality of honesty, directness, decision-making and leadership necessary to make this country great and successful again".
Business Secretary Lord Mandelson, who Mr Hague said had given "new meaning to the word chumps", was singled out during an attack on the Government's lack of accountability.
"The Prime Minister nobody ever elected has been kept in office by a deputy nobody in the country had voted for at all, making up a Government with the least moral or democratic authority to govern in our lifetime."