Walter F. Mondale and Frank R. Lautenberg return to the Senate, they will join a surprisingly large class of senators interruptus.

Barry M. Goldwater did it, leaving the Senate for his 1964 presidential bid, returning in 1969. So did Hubert H. Humphrey and Alben Barkley, both former vice presidents who came back to the world's most exclusive club, and other Senate fixtures from Henry Cabot Lodge to James O. Eastland. In the 19th century, when senators were still elected by state legislatures, politicians routinely moved in and out of the Senate.

Even since 1913, when the direct election of senators began, 33 senators have served nonconsecutive terms, according to the Senate Historians Office. But if Mr. Mondale prevails, he will stand out in one respect — the length of time between terms, with his 26 years beating even Andrew Jackson's 25-year gap.