The United States Senate has long been known as the World's Greatest Deliberative Body. Under the Constitution, it has a great deal of power and responsibility -- confirming judges, ambassadors and Cabinet officers, ratifying treaties and serving as a trial court in impeachment cases.

Lots of lower-level politicians would consider a term there the apex of a career. Plenty of youngsters with an interest in government dream of someday sitting in that exalted chamber. Several American statesmen with higher aspirations -- including Henry Clay, Robert Taft, Barry Goldwater and Ted Kennedy -- have found they could make history without ever leaving the Senate.

And then there is Fred Thompson, who saw it as a waste of his time. Back in 1998, only four years after being elected, he was out of patience. "I don't like spending 14- and 16-hour days voting on 'sense of the Senate' resolutions on irrelevant matters," he snorted. "There are some important things we really need to get on with -- and on a daily basis, it's very frustrating."

When his seat came up in 2002, the Tennessee Republican chose not to run again, which he now takes as a badge of honor. Asked recently by National Review to name his most important accomplishments in office, he replied, "You mean, besides leaving the Senate?"

It's a mystery why he ever sought the job. A former federal prosecutor, he had served as counsel to some Senate committees in the 1970s and 1980s, but in 1984, he passed up a chance to run for the seat held by his mentor, Republican Howard Baker of Tennessee. "The hassle factor is up," he complained, "and the pay is not."

Asked the following year if he might someday change his mind, he scoffed. In his younger days, he told People magazine, "I thought that standing on the Senate floor, engaging in a great debate and making a difference was the pinnacle of political activity. The more I've seen it, the less interested I am."

But for a while, he managed to overcome his sorrowful disillusion. In 1994, when a special election was called to fill the seat vacated by Al Gore, Thompson decided the Senate needed his talents, promised "a major shakeup to change the direction of our country" and came from behind to win.

On his arrival, the veteran actor was too famous to be just another senator. Barely a month after he was elected, he was asked to give the Republican response to President Clinton's address to the nation on the economy. When his party won control of the Senate, he became chairman of the Governmental Affairs Committee. He led an investigation into Democratic fund-raising.

In 1999, he took part in only the second presidential impeachment trial in American history (voting to acquit Clinton of perjury but convict him on obstruction of justice). He even got a few bills passed.

All this would be enough to keep most people engaged. And after the Sept. 11, 2001, attacks, Thompson said he'd run for re-election, because it was "not the time for me to leave." But before long, he concluded that the war on terrorism could be fought without his help.

Unlike Cincinnatus, he did not relinquish power to take up his plow, but rather to resume a lucrative career of lobbying and acting. Apparently, he found sitting through makeup sessions and waiting for his next take a more rewarding use of his time than voting on legislation.

Since he began toying with a presidential race, he has been accused of laziness. But no one who has made a living as a trial lawyer, as Thompson has, can be suspected of congenital sloth. In any event, Ronald Reagan and George W. Bush have proven that the presidency does not require a grueling work schedule.

The real question is whether Thompson really wants the office and all that comes with it for four long years. The Oval Office has its own quota of dull obligations -- attending state dinners, signing off on budgets, traveling to countries no tourist would ever choose, trying to remember the housing secretary's name -- which could be excruciating to someone with Thompson's low tolerance for tedium.

Americans, meanwhile, should wonder about entrusting the job to someone who has proven unable to sustain interest in the responsibilities that come with high federal office. In weighing his candidacy, they may find that the more they see of him, the less interested they are.