PERHAPS WITHOUT meaning to, Tom Ridge, the homeland security secretary, opened an unexpected can of worms last week when he warned of a terrorist attack designed to disrupt November's presidential election. His comments -- maddeningly lacking in detail, as always -- immediately suggested a number of nightmare electoral scenarios. What if a debilitating attack takes place the day before a presidential election? Should the election be postponed? What if the attack only hits an electorally sensitive city -- Miami, say, or Cleveland? Can elections be canceled in one place alone? And if so, do local officials decide, just as local officials decided to postpone New York City's municipal elections after Sept. 11?

Those who want to prepare for any of these scenarios are limited by the Constitution, which states that the presidential election must take place on a single day across the country and that Congress is responsible for setting the date. Yet on Election Day, Congress will not be in session. State and local officials will be in charge. And the time for disputes is limited: According to the 20th Amendment, the new president must be sworn in by Jan. 20, 2005. If not, the speaker of the House becomes president.

If the law is tricky, the politics is trickier. In recent days, Mr. Ridge informally asked the Justice Department to look at what might happen if an election had to be postponed, leading to a few suspicious, even hysterical reactions, and talk of stolen elections. While it is appropriate for the administration to think out loud about how it should react, it must bend over backward to do so in a bipartisan, premeditated fashion, with no hint of politicization. The legislature, not the executive, is clearly the body that should deal with this issue.

If, that is, it is to be dealt with at all. Some conversation about this subject is useful, if only to help prepare Congress and the White House for an emergency. Some have discussed the possibility of appointing a commission with distinguished, broadly respected members such as former senators Bob Dole and George J. Mitchell to look into this question, an eminently reasonable idea. At the same time, powerful emotional and even political arguments exist for holding a presidential election on the day it was meant to be held, regardless of what happens and who is unable to vote, just as it was held during the Civil War and just as it would be held in case of a hurricane, flood, fire or other natural catastrophe. This is not merely because talking about it in advance encourages those who might carry out a disruption. It's also because a postponed election would not necessarily have any greater legitimacy than an election disrupted by a terrorist attack. Congress should think through the consequences of a disrupted election, but it should remain extremely wary of any scheme to hold a presidential election at any time other than the first Tuesday of November.