One of the more unpleasant but unavoidable tasks awaiting Congress when it reconvenes this week is passage of a perennial bill that members love to hate: the federal debt ceiling extension. The Treasury Department estimates that it's about three weeks away from hitting the current ceiling of $985 billion. In any case, because of the way the law is written, the legal limit on the debt will drop back to $400 billion on Oct. 1 unless Congress acts before then; that would tie the whole government in knots.

Treasury Secretary Regan, his aides say, has decided to bite the bullet and ask for a new ceiling that will break the $1 trillion barrier. Regan wants the ceiling to provide enough breathing room so Congress won't have to take up the question again for a full year. Treasury officials say Regan might ask for a new ceiling of $1,079,000,000,000 -- the figure proposed by the House in its most recent budget package.

The debt itself seems sure to top a trillion dollars within the next two or three months. That means that Ronald Reagan, of all people, will be the first president to preside over a 13-digit national debt.