Foreign Secretary Jack Straw says anti-war activists can be drawn back to vote Labour by other factors.
Jack Straw said Iraq opponents would return to Labour
He told BBC News that his party was trying to "reach out" to voters who had been driven away by the Iraq war.
Mr Straw said the same people cared about issues such as Kashmir, Darfur and a Palestinian state, which Labour would address but the Tories would not.
But the Lib Dems' Matthew Taylor, and the Tories' David Cameron said voters had lost trust in Tony Blair.
Mr Straw said opponents of the war would recognise the "overall success of this Labour government".
But he said: "There has been a concerted campaign to try and use Iraq to undermine trust in the government and the prime minister."
Ex-foreign secretary Robin Cook said many former Labour voters, driven away by the war, could be brought back by Labour's pledges on international development.
Mr Cook - who resigned from the government in 2003 because of the Iraq war - said he would consider serving in a Blair administration again, but was not angling for a job.
The Liberal Democrats' parliamentary party chairman Matthew Taylor said the issue of Iraq was "coming up on the doorsteps".
He said for many it was the "moment at which they finally lost faith and trust in the prime minister".
Conservative policy co-ordinator David Cameron said his party still supported the decision to invade Iraq.
He said it had been right to get rid of Saddam Hussein, and that Britain had a responsibility to make sure democracy took hold in the country.
But he said that Tony Blair had not been straight with the British people and had a "huge problem with trust".
Mr Cameron said "sporadic and patchy" evidence about Iraq's weapons programmes had been portrayed as "detailed, authoritative and extensive" by the prime minister.