Menzies Campbell will today attack the "cosy consensus" between Labour and the Conservatives.
Sir Menzies will use his keynote speech to the Liberal Democrat conference to present the party as the only alternative to the centre-right consensus offered by the dominant two parties.
The Lib Dem leader will claim Gordon Brown has failed to offer the change many voters were looking for and is instead fighting with David Cameron over who can claim to be the "heir to Blair".
Only the Liberal Democrats can offer voters real choice on a range of issues including tax, education, the environment, criminal justice and more, he will argue.
He will say: "What our country needs is a political party that's prepared to take the lead and speak the truth, because on so many of the major political issues there is a two-party consensus - comfortable, cosy and complacent."
British politics is not "two against one" holding the government to account, but "one against two," he will tell delegates.
In a briefing to reporters ahead of the leader's speech Ed Davey, Sir Menzies' chief of staff, said the Liberal Democrats would set out their progressive liberal alternative in a policy heavy speech.
The Liberal Democrats will set out their attack on Mr Brown, who Mr Davey said had failed to move away from his predecessor's policies on Iraq, civil liberties, nuclear power, pensions and the environment.
They will argue David Cameron is not offering an alternative and instead fighting over the same narrow territory with Mr Brown.
Despite persistent poor approval ratings in the opinion polls, the Liberal Democrats insist the public consensus supports them when voters are asked to consider the issues rather than the politicians.
Mr Davey said Sir Menzies would address every issue head on, including the question of his leadership.
He is set to renew his pledge to lead the party into the next election and beyond, despite rumblings the party would be rejuvenated by a younger leader.
Mr Davey said the Liberal Democrats are not ruling out an early election, insisting the party is ready to go to the polls.