Scott McClellan is the John Dean of the Bush-Cheney administration — an in-the-know White House aide who has begun to provide the details of high crimes and misdemeanors committed by his bosses.

Like the former Nixon administration lawyer who started talking about a “cancer on the presidency,” the former White House spokesman has revealed dramatic details about wrongdoing on the part of George Bush, Dick Cheney, presidential and vice-presidential aide Scooter Libby and political czar Karl Rove.

So the fact that McClellan has agreed to testify under oath before the House Judiciary Committee next week is a big deal.

How big?

That depends on Congress and the media.

What McClellan will talk about goes to the heart of the matter of a lawless administration. When a White House insider agrees to deliver testimony to Congress about whether the Vice President of the United States ordered him to lie about a plot to discredit administration critic Joe Wilson, that’s a big deal. When a longtime aide to the president is prepared to tell a House committee that the commander-in-chief admitted to ordering the release of classified information as part of that plot, it is an even bigger deal.

But will members of the House Judiciary Committee recognize McClellan’s testimony for what it should be: the opening of an examination of impeachable offenses committed by the president, the vice president and their inner circle? Or will the key congressional committee — under pressure from a Speaker of the House who arbitrarily determines that the checking and balancing of the imperial presidency is “off the table” — approach its essential duties as a less-than-equal branch of government? (Ohio Congressman Dennis Kucinich proposed impeaching Cheney last year, and on Monday he introduced articles of impeachment against Bush. But only a handful of Judiciary Committee members, led by Florida Congressman Bob Wexler, have been willing to call for the opening of impeachment hearings.)

Will a free press that prefers to cover the sport of political campaigning rather than serious issue of how the current government has subverted the Constitution bother to put McClellan’s testimony in to context? For instance, will broadcast and cable networks note that the abuses of authority detailed by McClellan parallel the abuses that formed the underpinning for the third article of impeachment that a previous House Judiciary Committee lodged against Richard Nixon?

Every indication is that McClellan will do his part to break down the Bush-Cheney administration’s stonewall.

In his just-published book, What Happened: Inside the Bush White House and Washington’s Culture of Deception, the veteran Republican operative suggests that Cheney had pressured him to provide deceptive information about the role administration aides had played in blowing the cover of Wilson’s wife, CIA agent Valerie Place.

McClellan has stated publicly that Bush and Cheney “directed me to go out there and exonerate Scooter Libby.”

McClellan’s book and his statements led Judiciary Committee Chairman John Conyers, D-Mich., to invite the former White House spokesman to testify “concerning reported attempts to cover up the involvement of White House officials in the leak of (Plame’s identity).”

McClellan agreed to appear June 20 and his lawyer, Duke University law professor Michael Tigar says, “He has agreed to testify in the same spirit that he wrote the book… Mr. McClellan is available to tell what he knows.”

When America was a functioning constitutional republic, testimony of the sort that McClellans appears to be prepared to deliver would have been all that was required to get the Congress and the media to begin the long-delayed process of checking and balancing a lawless White House.

The questions that Scott McClellan’s testimony will raise are many.

But none will be more central than this: Is American still a functioning constitutional republic?