IN JUNE of 1863, Abraham Lincoln wrote a letter to Erasmus Corning, who had sent him the resolutions of the Albany Democratic convention censuring the Lincoln administration for what it called unconstitutional acts, such as military arrests of civilians in the North. This letter remains the best articulation of the problems that a democratic republic faces when confronted by a crisis that threatens the very existence of that republic.

The essence of Lincoln's argument was that certain actions that are unconstitutional in the absence of rebellion or invasion become constitutional when those conditions exists--in other words, "that the Constitution is not in its application in all respects the same in cases of rebellion or invasion involving the public safety, as it is in times of profound peace and public security."

This past Saturday, President Bush issued his equivalent of the Corning letter. In his weekly address to the nation, the president, speaking live from the Roosevelt Room, addressed the failure of the Senate to renew the Patriot Act, which passed that body 98-1 in the wake of 9/11, and forcefully defended his actions in authorizing the National Security Agency "consistent with U.S. law and the Constitution, to intercept the international communications of people with known links to al Qaeda and related terrorist organizations."

He pointed out that the Justice Department and NSA's top legal officials had reviewed the activities permitted under this authorization and that the executive branch had briefed congressional leaders of both parties more than a dozen times on this authorization and the activities conducted under it.

"This authorization is a vital tool in our war against the terrorists," the president said. "It is critical to saving American lives. The American people expect me to do everything in my power under our laws and Constitution to protect them and their civil liberties. And that is exactly what I will continue to do, so long as I'm the president of the United States."

The president faces a dilemma that was expressed by James Madison in a letter to Thomas Jefferson: "It is a melancholy reflection that liberty should be equally exposed to danger whether the government have too much or too little power." Lincoln addressed this dilemma during his speech to a special session of Congress after Fort Sumter. "Is there," he asked, "in all republics, this inherent, and fatal weakness? Must a government, of necessity, be too strong for the liberties of its own people, or too weak to maintain its own existence?"

THROUGHOUT THE HISTORY of the American republic, there has been a tension between two virtues necessary to sustain republican government: vigilance and responsibility. Vigilance is the jealousy on the part of the people that constitutes a necessary check on those who hold power, lest they abuse it. As Thomas Jefferson wrote, "[I]t is jealousy and not confidence which prescribes limited constitutions, to bind those whom we are obliged to trust with power."

But while vigilance is a necessary virtue, it may, if unchecked, lead to an extremism that incapacitates a government, preventing it from carrying out even its most necessary and legitimate purposes, e.g. providing for the common defense. "Jealousy," wrote Alexander Hamilton, often infects the "noble enthusiasm for liberty" with "a spirit of narrow and illiberal distrust."