Americans concerned with the balance of constitutional powers should demand that light be shed on a murky situation.

A little-discussed presidential directive adopted in recent weeks would, in effect, give the president almost complete power in the event of a "catastrophic emergency."

President Bush issued the document May 9, under the twin headings of National Security Presidential Directive 51 and Homeland Security Presidential Directive 20, with the subject line "National Continuity Policy."

The idea is to assure the smooth workings of government in the event of a disaster — but this function already is served by the National Emergency Act. That law gives the president authority to declare a national emergency but requires that the White House transmit the proclamation to Congress, which could rescind the authority if the president acts inappropriately.

The new directive appears to supercede the law. Its stated aim is to preserve a constitutional government, but it does so by pointedly placing the president in charge of all three branches.

Someone, please: Make sense of that contradiction.

The document even acknowledges that each branch already has the duty of ensuring its own continuity during a disaster. If that's the case, why does the president and key members of his staff need to be in charge of all three branches?

The most disturbing aspect of all this is the vague descriptions of the essential terms. A "catastrophic emergency" is defined as "any incident, regardless of location, that results in extraordinary levels of mass casualties, damage or disruption severely affecting the U.S. population, infrastructure, environment, economy or government functions."

It's anyone's guess how that criteria might be construed. And so far, a guess is the best option, since the White House has declined comment to the few who've asked.

So it's time for Congress to press for an explanation of what, in the absence of a clear statement, appears to be a naked power grab.