Charles Kennedy has put the defence of civil liberties at the heart of the Liberal Democrat election campaign.
Mr Kennedy said there was no 'glass ceiling' to the party's ambitions
Winning the issue was key to fulfilling the party's aim of "breaking the mould" of British politics, he said.
"We go back to Parliament to defend liberty; we go into an election to promote liberty," he told the party's spring conference in Harrogate.
Opposing the government's anti-terror legislation, which the conference agreed as policy, would be central.
The government says it needs the laws, which would bring in house arrest for some terrorist suspects, to prevent terrorist atrocities.
But a Lib Dem emergency motion dubbed the powers "unlawful, illiberal and ineffective", and called for tighter safeguards to be imposed.
The Prevention of Terrorism Bill dealt with "issues that go right to the heart of the civil liberties and the political basis of our country", he said.
The party wants the control orders, set to be introduced by the Prevention of Terrorism Bill, to be placed in the hands of judges not politicians.
In his final speech to the spring conference, he shared memories of the so-called Gang of Four who quit Labour to set up the SDP - which eventually joined with the Liberals to form the Lib Dems.
"They worked for the re-alignment of British politics. But they were up against it in terms of the system; they were up against it in terms of the opportunity."
On Saturday he told delegates the Lib Dems would present themselves as "the real alternative" in the general election campaign.
Unveiling the slogan, he said there was no "glass ceiling" to its ambitions.
Labour had abused the public's trust while the Tories had failed to oppose them, he said.
Mr Kennedy also reiterated his party's plan to impose a 50% income tax rate on earnings over £100,000 a year.
The money would be used to help pay for key policies such as abolishing university tuition fees, scrapping council tax in favour of local income tax, and providing free care for the elderly, he said.
Labour and the Tories claim the sums do not add up, and that working families would be hardest hit.
But Lib Dem Treasury spokesman Vince Cable insisted all the party's proposals were fully costed.
He told BBC One's Breakfast with Frost programme the average family would be £450 better off if the council tax was replaced with a local income tax.
But he acknowledged that three out of 10 wealthier households would have to pay more.
Delegates also backed a motion calling for British troops to be pulled out of Iraq by the end of the year.
It urges the government to draw up an exit strategy which would see UK forces in Iraq return home by the end of 2005.