As a candidate for lieutenant governor in 1982, John Kerry assured the voters of Massachusetts that he wasn’t seeking the position as a mere “stepping-stone” to higher office. But just one year into his four-year term, he announced his candidacy for the US Senate seat that Paul Tsongas was vacating because of illness.

Few people held Kerry’s broken commitment against him. In part that was because nobody had believed it in the first place (all candidates for lieutenant governor seek the position as a stepping-stone). But it was also because everyone knew what Kerry knew: If he passed up the chance to run for the position Tsongas was relinquishing, it might be years before it opened up again. So Kerry jumped into the Senate race and won. Sure enough, the seat has been occupied ever since.

For nearly 28 years Kerry has been a senator, and in all that time no Massachusetts Democrat has ever seriously challenged him in a primary. (He faced token opposition from a little-known Gloucester lawyer in 2008). Yet once speculation began that President Obama might name Kerry to a Cabinet post, three Democratic congressmen – Edward Markey, Michael Capuano, and Stephen Lynch – quickly let it be known that they were interested in taking his place, raising the likelihood of a knock-down primary.

A Senate bid by any of them would undoubtedly trigger in turn a lively primary fight for the House seat (or seats) being vacated. Otherwise, none is likely to face more than weak opposition for his party’s renomination – especially not from incumbents lower down on the food chain, hoping someday to move up. The last time a member of the Massachusetts congressional delegation lost a primary battle was 20 years ago, when Marty Meehan of Lowell ousted Concord’s Chet Atkins. Before that it hadn’t happened since 1970.

What’s true of congressional incumbents is just as true of the mayoral variety.

A slew of Boston Democrats is reportedly poised to run for the city’s top job next year – but only if five-term incumbent Thomas Menino bows out. The fact that the mayor suffers from multiple ailments, that he is now hospitalized “indefinitely,” that he hasn’t set foot in City Hall for over a month – none of that changes political reality: As long as he chooses to be mayor of Boston, the job is his to keep.

When former Mayor Ray Flynn resigned to become US ambassador to the Vatican in 1993, a vigorous free-for-all to choose his successor featured some of the most able figures in Boston life. That was a healthy, competitive contest. There won’t be another one like it until Menino departs. Until then, most mayoral hopefuls will simply bide their time. Menino will go through the motions of running for re-election, brushing past a quadrennial opponent that everyone knows doesn’t have a chance. Give a Boston mayor the boot? Voters haven’t done it since they expelled James Michael Curley, a convicted felon, in 1949.

Ours isn’t the only part of the country where incumbency-worship runs deep. West Virginia sent Robert Byrd to the US Senate for 51 years, and Daniel Inouye has represented Hawaii in Congress since it became a state in 1959. Charleston, S.C., has had the same mayor since 1975. No matter how unpopular Congress is said to be, more than 90 percent of House members seeking re-election generally keep their seats; in that respect Nov. 6 was absolutely typical.

Yet American politicians didn’t always assume that incumbency was meant to be for life. Most of Kerry’s Senate predecessors served one or two terms and moved on; the endless reigns of senators like Ted Kennedy (46 years) and Henry Cabot Lodge (31 years) were historical anomalies. Yes, there is always the possibility of electing someone so exceptional that his talents and experience make him irreplaceable. But the odds are overwhelmingly against it. Far better for officials to come and go, serving a spell in government, then heading back to real life.

“Representatives ought to return home and mix with the people,” Rhode Island’s Roger Sherman argued during the Constitutional Convention in 1787. “By remaining at the seat of government, they would acquire the habits of the place, which might differ from those of their constituents.”

George Washington could have been president for life, but he voluntarily stepped down after two terms. He could be trusted with power precisely because he could let it go. Politicians today can’t bear the thought of giving up the power with which we cloak them. And we, to our discredit, are rarely prepared to take it away.

(Jeff Jacoby is a columnist for The Boston Globe. His website is www.JeffJacoby.com).