Thirteen years ago, the United States was changed in a profound way. A major attack on the homeland by a nonstate foreign actor had never happened before, and the consequences — thousands of deaths, landmarks destroyed and damaged — were terrifying. The policy repercussions, from the Patriot Act to the Guantanamo Bay prison to wars in Afghanistan and Iraq to airport security measures to creation of the Department of Homeland Security to reorganization of the intelligence agencies, were even more far-reaching.

In part as a result of these changes, the United States has gone for 13 years without another devastating terrorist attack. But as the rise of the Islamic State in Iraq and the Levant underscores, the threat has not disappeared. As time passes, the access of terrorist groups to powerful weapons, and their sophistication at probing our defenses and recruiting potential attackers, continues to increase.

While the United States has taken many steps to stay one step ahead of terrorists, there is one area in which multiple efforts to protect us have been sidestepped, ignored or abandoned. That area is in continuity of government — ensuring that our fundamental institutions, including Congress, the presidency and the judiciary, are able to immediately reconstitute themselves if shattered by a terrorist attack.

The danger has been apparent since Sept. 11, 2001. United Airlines Flight 93, the plane that crashed in rural Pennsylvania because of the actions of its brave passengers, was likely headed for the Capitol. If it had not been delayed on takeoff from Newark by 45 minutes, the passengers would not have known they were on a suicide mission, and the plane probably would have hit the Capitol around the same time American Airlines Flight 77 struck the Pentagon — causing massive damage and potentially leaving Congress without the constitutionally mandated quorum of half its members required to do business.

Coincidentally, that same morning the Judicial Conference of the United States was meeting at the Supreme Court, a stone's throw from the Capitol. The core of the federal judiciary was there, with Chief Justice William Rehnquist presiding. Conceivably, a direct hit on the cast-iron Capitol dome could have created devastating damage to the court building, decimating the judiciary.

When it became clear that the devastation was indeed a terrorist attack, Air Force One took off from Sarasota, Fla., with the president on board, and headed west to heavily protected air bases, while the vice president was hustled into an underground bunker at the White House to safeguard the line of presidential succession. But in the aftermath of the attacks, it became clear that the Presidential Succession Act of 1947 never contemplated an attack carried out with no notice (unlike during the Cold War, when there would be crucial time after nuclear missiles were launched from Siberia) while large numbers of those at the top of the three branches of government were in close proximity. An attack with a suitcase nuclear bomb at a presidential inauguration would be particularly damaging without better rules for government succession in place.

Weeks after the Sept. 11 attacks, I joined with Tom Mann of the Brookings Institution to create a Continuity of Government Commission, co-chaired by former White House counsel Lloyd Cutler and former senator Alan Simpson, with distinguished scholars, former lawmakers and Cabinet officers as members. We issued three reports — one each on Congress, the presidency and the Supreme Court. We recommended a constitutional amendment to allow emergency interim members of Congress to replace those dead, incapacitated or missing after a massive attack, until the incapacitated could recover and the dead could be replaced in meaningful special elections. This would enable Congress to get up and running legitimately within days after an attack, preventing the need for martial law at the worst possible time.

We recommended a thorough revamping of the Presidential Succession Act to bring it up to date and erase a set of unintended anomalies. We also recommended creation of an interim Emergency Court of Appeals, consisting of the chief judges of the various circuit courts of appeals and the remaining justices of the Supreme Court, in case an attack reduced the Supreme Court below its statutory quorum requirement of six. Having a functioning court of last resort would be critical to settle any questions about the legitimacy of actions by a damaged Congress or about presidential succession.

We had allies across party lines in both houses. But little of consequence happened. There was active opposition to serious action from House Judiciary Committee Chairman James Sensenbrenner, R-Wis., and House Rules Committee Chairman David Dreier, R-Calif., on the grounds that there should never be an appointed member of the House, no matter the circumstance. But the bigger problems were human nature and lethargy. Creating temporary members of Congress to replace the dead meant requiring members to contemplate their own demise. Congressional leaders of both parties always found other, more immediate problems to deal with.

This was nothing new in U.S. history. Previous revisions of presidential succession, for example, took multiple disasters of presidential assassination to precipitate action. But that is no excuse.

Thirteen years later, the threat is, if anything, greater than it was after Sept. 11. The need to preserve and protect continuity in our constitutional institutions is critical. We are way past time for leaders of both parties to do their duty.

Norman Ornstein is a resident scholar at the American Enterprise Institute. He wrote this article for the Washington Post.