All those incumbent congressmen must be pretty smug this year. What with the economy humming along, and voters so docile that even the rake in the White House gets high approval ratings, what could possibly go wrong between now and November?

Lots. GOP strategists Mike Murphy and Alan Philp argue that incumbent senators, especially, should be running hard and avoiding complacency. They point to a curious pattern they dub "the Curse of '68," which recurs this year.

With six-year terms, only one-third of the 100 senators run for reelection in any election year. And the six-year cycle starting with the class of 1968 has been volatile. In 1974, 1980, 1986, and 1992, an unusual number of incumbents have been unseated. Thirty incumbents have lost their reelection bids in those elections -- that's an average of six incumbents losing each time. By contrast, the average losses for the cycle starting with the class of 1970 are 3.8 seats; for the class of 1972, it's 3.4 losing incumbents on average.

Some of the most famous post-World War II names in American politics have been swept out of office by the Curse of '68: Herman Talmadge, a fixture in Georgia politics, lost to Republican Mack Mattingly in 1980; George McGovern was upset by South Dakota Republican James Abdnor that same year. Each of those winners would himself lose six years later.

Is this an example of astrology politics? Hardly. But it's one more reason for Carol Moseley-Braun, Barbara Boxer, Patty Murray, Kit Bond, Al D'Amato, and a couple of "safe" senators nobody's yet thought much about to look over their shoulders.