Vermont Senator Patrick Leahy was asked in December whether President Bush should be “worried” that the liberal Democrat would soon take over as chair of the powerful Senate Judiciary Committee. “No, no,” Leahy replied. “No, he shouldn’t be worried. He should be terrified.” The anecdote, which Leahy has delighted in repeating to Democratic audiences in his home state, highlights the most significant reality of the new Congress.

Yes, Democrats will be in control of the Senate for the first time since 2003 and of the House for the first time since 1995. But the majorities in both chambers are relatively narrow, and the executive branch is still in the hands of conservative Republicans who, for all their polite references to bipartisanship, are more than ready to take advantage of divided government to prevent the course correction that Americans voted for on November 7. So why would Bush be terrified by Leahy’s chairmanship, or that of other Democrats who will be taking charge of key committees and subcommittees? Because the most important work of this Congress will be the restoration of a system of checks and balances–with the power to hold the executive branch to account–that even Republicans like Senator Chuck Hagel admit ceased to function during the first six years of the Bush presidency.

Committee and subcommittee chairs have the power not merely to advance legislation and review appointments but to schedule hearings, initiate investigations, compel executive branch members to release information and, where necessary, subpoena testimony from White House aides. Bush and to an even greater extent Vice President Cheney do not fear a Congress that seeks to raise the minimum wage or tinker with tax cuts for the rich. They can deal with those fights–indeed, they may even relish them for purposes of political positioning. But this most secretive of administrations fears investigation as a vampire does sunlight.

Bush and Cheney have reason to be concerned. House Speaker Nancy Pelosi may say that proposals to impeach the President and Vice President for abusing their offices and the Constitution are “off the table,” and Senate majority leader Harry Reid may still be determined to block efforts by Senator Russ Feingold to censure Bush for approving warrantless wiretapping. But committee and subcommittee chairs have broad leeway, and many of the new chairs intend to tear away the veil of secrecy that until now has prevented examination of the manipulation of intelligence before the invasion of Iraq, profiteering by companies associated with Cheney and others, illegal spying, the targeting of critics for retribution and other abuses of power.

Impeachment and censure are best seen as organic processes that begin with investigations, which inspire public outrage. It is this outrage that ultimately emboldens members of Congress to place constitutional remedies on the table. There are no guarantees that this new Congress will get close enough to the truth of the Administration’s misdeeds to inspire the application of the most severe of sanctions. But the right people are in the right places to advance the process. Consider that Feingold, the only senator to vote against the Patriot Act and the senator most concerned about checking the President’s power to wage war without a proper declaration from Congress and to spy without a warrant, will now chair the Constitution subcommittee of Leahy’s Judiciary Committee. Feingold, who says the censure option remains on his table, plans an ambitious set of hearings on issues related to the war and domestic eavesdropping.

His counterpart on the Constitution subcommittee of the House, Jerry Nadler, is equally prepared to hold the President to account. Last year, when he sought to force Attorney General Alberto Gonzales to submit documents to Congress pertaining to warrantless wiretapping, Nadler said, “The President took an oath to ‘preserve, protect and defend the Constitution of the United States,’ and to ‘take care that the laws be faithfully executed.’ When he acts outside the limits set by the Constitution and contrary to the law, he engages in a criminal conspiracy against the United States, against the separation of powers, one of the chief pillars supporting our liberties, and against those liberties.” Nadler went on, “This is a direct challenge to us. It is our responsibility, as members of the House of Representatives, to protect American liberties by investigating the President’s usurpations of power and to determine whether they constitute ‘high crimes and misdemeanors’ in the constitutional sense. It would be a terrible dereliction of duty if we were to disregard this responsibility.”

Nadler now finds himself positioned to perform oversight duties that had been stymied by Republicans. So, too, does House Government Reform Committee chair Henry Waxman. The new head of the chamber’s chief investigative committee has a long list of issues on which he says the last Congress failed to conduct meaningful oversight. Item 1: “Congress has not investigated whether White House officials misrepresented intelligence on Iraq to make the case for war.” Item 2: “Congress has not investigated the responsibility of senior Administration officials for Abu Ghraib and abuses of detainees.” The list includes references to the many instances of abuse associated with Cheney’s former firm Halliburton, which will face scrutiny not just from Waxman and his investigators but from the dozens of accounting detectives under the direction of House Appropriations Committee chair David Obey and investigators associated with the House Armed Services Committee, whose new chair, Ike Skelton, says one of his first acts will be to resurrect the subcommittee on oversight and investigations that Republicans had disbanded.

Waxman is likely to be in the forefront, however, as he is widely respected as the chamber’s savviest and most unrelenting inquisitor. The California Democrat will not give Republicans a talking point by immediately dispatching the “blizzard of subpoenas” that they have predicted. Rather, he will marshal his evidence, dusting off the more than 2,000 reports prepared by his staff since he became the ranking Democrat on the committee a decade ago and filling in the blanks that were left open during the years of Republican hegemony–first with official requests from a now empowered committee chair and then, if necessary, with the full force of the Congress to compel cooperation.

“The most difficult thing will be to pick and choose,” he says of his agenda. But Waxman has no doubt about the point of his inquiries: He will not rest, he says, until “Congressional oversight is reinvigorated and the checks and balances on which our system of government depends are restored.”

In the process, a President and Vice President who for six years have governed without consequence may well learn that a Congress that takes the Constitution seriously can force the White House to do the same.