(FORTUNE Magazine) – It is Jan. 20, 2005. A suitcase nuclear bomb explodes on the National Mall during an Inaugural ceremony. The incoming President and Vice President, the incoming cabinet (none yet sworn in), the entire congressional leadership and most of the rank-and-file, and the whole Supreme Court are lost. All those in the line of succession to the presidency are gone; it is unclear who will be in charge. Trying to be helpful, governors, generals, surviving members of Congress, perhaps the outgoing Secretary of State, announce their availability. But for days or weeks there is no official body or person to sort out competing claims.

Eventually a small group of surviving Congresspeople announces that it constitutes a quorum; a literal handful elect a speaker, who becomes acting President under the authority of the Presidential Succession Act of 1947--for a full four years, with the power, among other things, to nominate all nine Supreme Court justices for life.

Scary, huh? A few years ago that horrific scenario would have been the stuff of a Tom Clancy novel. Since 9/11 it has gone from the fanciful side of the ledger to the real-world side--one of a series of potential threats never imagined by the framers of the Constitution or by subsequent lawmakers who wrote the laws governing presidential succession.

The Constitution, of course, created a Vice President to step in if the presidency were vacant. But it left the question of succession beyond the Vice President up to Congress, specifying only that successors be "officers" of the United States. Three times since the Republic began--in 1792, 1886, and 1947--Congress devised succession plans. The current system, in place for the past 57 years, puts the speaker of the House of Representatives and the president pro tempore of the Senate in line after the Vice President, followed by the cabinet in order of the creation of the office. (The sequence goes State, Treasury, Defense, Attorney General, and onward.)

Every year at the State of the Union address, network anchors point out that a cabinet member is absent from the Capitol and in an undisclosed location, to ensure that an attack won't wipe out the entire line. Otherwise, unless crisis frightens the nation, presidential succession is largely relegated to SAT preparation. Political mavens had to dig out the civics textbooks when Richard Nixon faced impeachment, especially after Vice President Spiro Agnew resigned. Musing about a President Carl Albert (then speaker of the House), President James Eastland (president pro tem of the Senate), President Henry Kissinger (nope, ineligible because of foreign birth), or President William Simon (Treasury Secretary) became a sport du jour. The textbooks came in handy again when President Reagan was shot in 1981 and went into emergency surgery. With Vice President George H.W. Bush in the air and out of secure contact, the nation was electrified (and hardly reassured) when Secretary of State Alexander Haig assured the public that he was in charge. (It sounded like a coup attempt, but his statement turned out to be more a poor choice of words and ignorance of the law.)

Succession law changes in response to events. Today's rules were established in 1947 at the instigation of Harry Truman. When he took office upon F.D.R.'s death in 1945, the line of succession consisted of the cabinet secretaries. Truman felt vulnerable; the world was still at war, yet he had no Vice President and no way to fill that slot for nearly four years. Next in line was Secretary of State Edward Stettinius (whom Truman didn't warm to and would soon replace). Truman believed that having unelected cabinet members leading the succession was undemocratic. Within two months after assuming the presidency, he announced a push for reform. He went back to a 1792 principle of putting senior members of Congress in the succession. Because powerful Sam Rayburn was House speaker, he placed him first in line, followed by the president pro tem of the Senate and then by the cabinet.

Fifty-seven years later, especially in light of 9/11, succession deserves another look. Constitutional scholar Miller Baker calls the 1947 act "perhaps the most poorly designed statute in the entire United States Code." Eminent Yale Law School professor Akhil Amar says it is "a disastrous statute, an accident waiting to happen." What are its big flaws? Most constitutional experts believe that having members of Congress in the line of succession is unconstitutional and unwise. Since the speaker and the president pro tem can be members of the other party, a presidential tragedy can cause an abrupt and unintended shift in party control and policy direction. What's more, the president pro tem is the Senate's most senior majority member--which is to say, a really old person. (Consider Strom Thurmond, who at age 98 was still president pro tem and third in line for the Oval Office.)

Simply removing congressional leaders from the line and having the cabinet as the designated successors is no panacea. Cabinet secretaries in many cases are chosen for reasons of geography, ethnic balance, friendship with the President, or narrow expertise, which have little to do with qualifications to be President. Moreover, it is clearly risky in the age of terrorism to have every person in the line of succession reside in Washington.

There are three ways to clean up the law and respond to current threats. First, remove congressional leaders from the line of succession. Second, ensure that the four top cabinet officers are confirmed by the Senate and sworn into their posts on the morning of the Inauguration--then whisk one or more from the city for the day. Third, let the President nominate a series of individuals to become designated successors, who would then be confirmed by the Senate as "officers" of the United States. Presidents would be encouraged to pick people around the country who have demonstrable qualifications to step into the Oval Office--ex-Presidents and Vice Presidents, perhaps, and prominent governors, statesmen, and stateswomen. As officers, they would be briefed regularly on security and other issues so they would be prepared for leadership.

Focusing on succession is like writing a will. Smart people often use remarkable mental gymnastics to avoid thinking about their demise or grappling with delicate issues that need to be resolved in advance. It is easy to rationalize that you can deal with it later or that the problems will somehow resolve themselves. Happily, some members of Congress, including Senators John Cornyn of Texas, Trent Lott of Mississippi, and Chris Dodd of Connecticut, have shown a recent willingness to take on presidential succession. The immediate threat may be slight--but it is real. Better sooner than too late.

NORMAN ORNSTEIN is a resident scholar at the American Enterprise Institute and a senior counselor to the Continuity of Government Commission.