Liberal Leader Paul Martin dropped the most surprising news of Monday's leaders' debate, revealing an unexpected plan to amend the Charter of Rights and Freedoms to bar federal use of the notwithstanding clause.

In the name of defending the Charter, Martin proposed the Amendment to End Federal Ability to Invoke Notwithstanding Clause.

The details were quickly posted on the Liberal website, which billed it as "the boldest defence of rights and freedoms in Canada since the creation of the Charter of Rights and Freedoms in 1982."

As it now stands, the clause gives the government the power to overrule court rulings on Charter issues. It has never been used federally.

• More coverage of the debate
• David Akin blog item on notwithstanding clause
• Transcript and video

Martin revealed his plan during the debate, and challenged Harper to also commit to banning federal use of the clause. Harper ignored the goading.

The amendment would ban federal Parliament from using the clause, but would still grant the right to provincial governments.

The Liberals have suggested throughout the campaign that Harper would be forced to use the notwithstanding clause in order to make good on his promise to bring the issue of same-sex marriage back to Parliament for a free vote.

Harper has denied he will invoke the clause, and after the debate he played down Martin's announcement, calling it a "channel changer" designed to distract voters from scandals that plague the Liberal Party.

"It's a constitutional policy that was done in the middle of a national debate, in the middle of a campaign. I think it's difficult to take this seriously," said Harper.

Martin, however, spoke to reporters after the debate and defended his plan. He said it is intended to further enshrine the rights of minorities and religious groups.

"In Canada today and Canada as it may evolve, I believe we're going to want minority rights to be protected in case in any circumstances a prime minister was going to decide what rights he would and wouldn't protect, and I think that's fundamentally wrong," Martin said.

Earlier Monday, the Liberals were questioning Tory MPs' positions on same-sex marriage, pointing to a 2003 U.S. radio interview and video from last year where current candidates spoke out against same-sex marriage.

Martin's announcement seemed aimed at reigniting voter fear of a hidden Tory agenda.

Harper also revealed his intention to amend the Charter in order to protect property rights. Although it's the first time it has been brought up during the debate, it's a longstanding Conservative value, dating back to the former Alliance Party.

"This is a Conservative principle," Harper said when questioned by reporters. "We feel that the population must have the right to own property, to have a legal process in cases of expropriation, and we feel it's necessary to compensate people in such cases, and I feel those priorities should be protected by the constitution."

However, Harper added that he has no intention to pursue the change without the consensus of the provinces.

Former Liberal cabinet minister Brian Tobin appeared on CTV's Countdown following the debate, and said Harper's plan would require unanimity in Parliament, and would be an undertaking on the scale of the Meech Lake Accord.

Martin's plan for constitutional change, on the other hand, could be passed through a normal Parliamentary vote, according to Tobin.