It is Inauguration Day, and al-Qaida detonates a small nuclear device on Pennsylvania Avenue. The explosion kills or incapacitates the president-elect, the vice president-elect, his unconfirmed Cabinet and most members of Congress.

Who is in charge?

A blue-ribbon panel sketched that nightmare scenario Wednesday and proposed some solutions.

“What literally was the stuff of Tom Clancy novels is now something that we have to deal with,” said Norman Ornstein of the American Enterprise Institute.

In fact, it nearly happened on Sept. 11, 2001. United Flight 93 reportedly was headed for the U.S. Capitol when passengers fought its terrorist hijackers and forced the plane to crash in Pennsylvania.

Though most officials have focused since then on protecting the executive branch, Ornstein, Rep. Brian Baird, D-Wash., and others have recognized that terrorists would create a serious problem if they could wipe out a majority of the Senate and the House.

The reason: While most governors have the power to name new senators, only the voters can replace House members. That could take months.

Texans have had firsthand experience with the problem.

In 1993, appointed Sen. Bob Krueger took office a day after Sen. Lloyd Bentsen resigned to become treasury secretary. But in 1991, it took 68 days for Sam Johnson to be elected and sworn in after Rep. Steve Bartlett resigned to run for mayor of Dallas.

That is no problem in normal times. But it would be disastrous if terrorists were to kill or incapacitate most House members, since it takes both houses to approve almost everything except treaties and nominations.

Besides, if the president and vice president were killed, the speaker of the House would be next in line to be president — assuming there is a speaker.

That is why the commission decided to focus on Congress in its first report, which proposes a constitutional amendment giving lawmakers authority to fill mass vacancies.

It stressed the need for fast action. As former Rep. Lynn Martin, R-Ill., put it, “None of us really think it’s going to happen. But we can’t guarantee it.”

The panel outlined two alternatives: a bare-bones amendment giving Congress authority to set rules to fill vacancies or a more complex measure with the details.

It sees governors filling House vacancies temporarily, as they do now with senators, current members drawing up a list of potential successors to ensure ideological continuity, or perhaps a combination of the two.

Panel member Ken Duberstein, a former White House chief of staff, said he is optimistic something can be done within the commission’s two-year target.

“That would be terrific — as long as we don’t have a catastrophe in the meantime,” he said.

Texas Sen. John Cornyn, who heads the subcommittee on the Constitution, said he would hold hearings, noting that “it will be far too late … after a catastrophe strikes the Capitol building.”

House leaders are forming a special joint committee to study this and related issues.

This wouldn’t be the first time Congress has updated the Constitution in such a way. It happened in the 1960s after President John F. Kennedy’s assassination left House Speaker John McCormack, D-Mass., and Senate President Pro Tem Carl Hayden, D-Ariz., in line to succeed President Lyndon Johnson.

The idea of the two aging congressional veterans becoming president spurred Congress to pass the 25th Amendment, creating a way to fill a vice-presidential vacancy or deal with possible presidential disability.

A decade later, the amendment was used twice in a year when bribery charges forced Vice President Spiro Agnew to resign and the Watergate scandal forced President Richard Nixon to quit.

Former House Republican leader Gerald Ford wound up as president and former New York Gov. Nelson Rockefeller as vice president, and the general consensus was that the amendment had served the country well.

It is a useful reminder that the time to deal with such problems is now, before terrorists strike again.

— Carl P. Leubsdorf is Washington bureau chief of the Dallas Morning News.