What if United Flight 93--the plane that crash landed near Pittsburgh--had crashed into the Capitol dome on Sept. 11 at 9:30 or 10 a.m.? Both chambers of Congress were filled with lawmakers that morning. Hundreds might have been killed or incapacitated.

The horror of such an attack would soon be punctuated by an additional horror--a Congress unable to meet or act for months, because of constitutional provisions that do not contemplate a disaster of this dimension.

The problem is twofold: First is a provision in Article I that defines the number of members in each body required to be present to pass laws as a majority. Parliamentarians define this quorum as a majority of those elected, sworn in and living. But a massive terrorist attack might leave sizable numbers alive but incapacitated. Even if it could operate with 30 or 40 members, imagine the strain on the system if Congress passed sweeping anti-terrorism laws, huge new appropriations, even a declaration of war, with most of the country unrepresented, or with a radically different partisan or regional makeup.

Members of Congress can be replaced and the membership replenished. In the Senate, where vacancies can be filled either by special elections or gubernatorial appointments, governors could step in quickly. But the Constitution says vacancies in the House of Representatives can by filled only by special election. The past six special elections to the House have taken between three and six months.

The possibility of months without a Congress, at the worst possible time, is unacceptable. The answer is a constitutional amendment to allow for governors to make interim emergency appointments. This would keep the basic structure our Founders envisioned in place but also account for a catastrophe the Framers could not have foreseen. Rep. Brian Baird of Washington has drafted such an amendment, allowing 90-day appointments if a quarter of the Congress is killed.

Congress’s leaders no doubt do not want to confront the possibility of their own demise. But it would be irresponsible to put off or ignore this issue.