It sounds as if Al Gore is about to deliver what could be not just one of the more significant speeches of his political career but an essential challenge to the embattled presidency of George W. Bush.

In a major address slated for delivery Monday in Washington, the former Vice President is expected to argue that the Bush administration has created a “Constitutional crisis” by acting without the authorization of the Congress and the courts to spy on Americans and otherwise abuse basic liberties.

Aides who are familiar with the preparations for the address say that Gore will frame his remarks in Constitutional language. The Democrat who beat Bush by more than 500,000 votes in the 2000 presidential election has agreed to deliver his remarks in a symbolically powerful location: the historic Constitution Hall of the Daughters of the American Revolution. But this will not be the sort of cautious, bureacratic speech for which Gore was frequently criticized during his years in the Senate and the White House.

Indeed, his aides and allies are framing it as a “call to arms” in defense of the Bill of Rights and the rule of law in a time of executive excess.

The vice president will, according to the groups that have arranged for his appearance — the bipartisan Liberty Coalition and the American Constitution Society for Law and Policy — address “the threat posed by policies of the Bush Administration to the Constitution and the checks and balances it created. The speech will specifically point to domestic wiretapping and torture as examples of the administration’s efforts to extend executive power beyond Congressional direction and judicial review.”

Coming only a few weeks after U.S. Representative John Conyers, the ranking Democrat on the House Judiciary Committee, introduced resolutions to censure President Bush and Vice President Cheney, and to explore the issue of impeachment, Gore in expected to “make the case that the country — including the legislative and judicial branches and all Americans — must act now to defend the systems put into place by the country’s founders to curb executive power or risk permanent and irreversible damage to the Constitution.”

Don’t expect a direct call for impeachment from the former vice president. But do expect Gore to make reference to Richard Nixon, whose abuses of executive authority led to calls for his impeachment — a fate the 37th president avoided by resigning in 1974.

Gore’s speech will add fuel to the fire that was ignited when it was revealed that Bush had secretly authorized National Security Agency to monitor communications in the United States without warrants. Gore will argue that the domestic wiretapping policy is only the latest example of the administration exceeding its authority under the Constitution.

With a Congressional inquiry into Bush’s repeated violations of the Foreign Intelligence Surveillance Act scheduled to begin in February — and with Bush already preparing to pitch an Nixon-style defense that suggests it is appropriate for the executive branch to violate the law when national security matters are involved — Gore will articulate the more traditional view that reasonable checks and balances are required even in a time of war. And he will do so in a bipartisan context that will make it tougher for Republican critics to dismiss the former vice president’s assertion that the Constitution is still the law of the land.

Former U.S. Representative Bob Barr, the Georgia Republican who served as one of the most conservative members of the House, plans to introduce Gore. Barr, an outspoken critic of the abuses of civil liberties contained in the USA Patriot Act critic who has devoted his post-Congressional years to defending the Bill of Rights, refers to the president’s secret authorization of domestic wiretapping as “an egregious violation of the electronic surveillance laws.”

Count on Gore, who has pulled few punches in the speeches he has delivered in recent months, to be at least as caustic.