She has a steely sense of discipline. He is relaxed, unpretentious. She has no nicknames. He has several. She has an eye for the dry policy paper. He likes a good spy novel. She has a practiced smile. He has an easy one. She is unfailingly formal in public. He relishes the informal aside. She’s married to Bill Clinton. He’s married to Hillary Rodham Clinton.

Commentators and analysts have spent weeks plumbing the differences between Mrs. Clinton and Rudolph W. Giuliani, and between Mrs. Clinton and Barack Obama, and between Mrs. Clinton and John Edwards. I’ve done it myself.

But for all that exertion, it only now has occurred to me that the most illuminating comparison might not be between Mrs. Clinton and the men who are competing with her to be the successor to George W. Bush. The most revealing comparison is instead between Mrs. Clinton and the man who preceded George W. Bush in the White House.

Much in common

People often say that the Clinton marriage is a merger, or a political partnership, and they may be right; only two people on Earth know, and they’re not saying. But what may be most relevant to voters contemplating the next president may be to view the Clinton marriage as a comparison.

On the surface, as different as the Clintons are, they also have much in common, besides having ferocious tempers and slightly wacko brothers. Both were marked by early religious experiences. Both are baby boomers, graduates of Yale Law School and early adherents to liberal Democratic politics. Both see their lives as a series of political passages en route to a series of political positions. Both were drawn into the signature political battles of the 1960s and were affected deeply by what happened to them before they turned 30. (Indeed, they are proof of the theory that how young you are isn’t nearly as important as when you were young.) But the differences are far more interesting.

Bill Clinton had none of the easy affluence of so many of his demographic cohorts. He fought for what he wanted, doing so in an atmosphere where he was the pioneer – the first to go to college, the first to see the broad horizon, the first to feel that his dreams could be realized, not merely repressed.

Hillary Clinton worked hard, too – nobody ever called her a slacker, which would be slander – but she did so in an atmosphere of sunny middle-class possibility. Her father went to Penn State (and played for the Nittany Lions football team), owned a business and personified the American dream that the young Bill Clinton read about but never experienced. Bill Clinton’s father figure was J. William Fulbright. Hillary Clinton’s father figure was her father.

Out front vs. in the background

Bill Clinton never wanted to be a behind-the-scenes power. If he sought power, it was visible power. He applied for a Rhodes Scholarship and got it, thereby acquiring the most shimmery emblem of destiny that the world offers. He went back to Arkansas and almost immediately ran for office.

Like her future husband, Hillary Rodham won admission to Yale Law, then as now slightly more prestigious than Harvard for being so much smaller, but when she went out into the world it was as staff – on the staff of the House committee contemplating the impeachment of President Richard M. Nixon, which was not like being on the staff of the Omaha city council, but staff nonetheless. Then she moved to Arkansas and helped Bill Clinton run for office.

Both were ambitious, but Bill Clinton sought to run the table outdoors while Hillary Clinton preferred to do so indoors. It is an important distinction. The risks for Bill Clinton may have been greater – it was he who was repudiated after his first term as governor, for example – but the work was probably harder for Hillary Clinton. As a behind-the-scenes operator in the governor’s mansion and as a partner in the Rose Law Firm, she did the difficult, adult work while her husband jawed with the good-ole-boys and stuffed down corn dogs at the state fair. In a moment of honest reflection, Bill Clinton would agree.

Much of that changed, of course, in 2000, when Hillary Clinton decided to run for the Senate herself. Indeed, though it is true that she had been involved with public issues all her life, it is also true that with the exception of her commencement speech at Wellesley College, her education initiative in Arkansas and her health-care plan in Washington, she had rarely taken a solo public role.

Now, of course, she is seeking the biggest solo public role in the world. She’s the first presidential spouse to do so, although none of her Wellesley classmates would have been surprised to learn that the president of the College Government Association in 1969 was the favorite to become the president of the United States in 2008.

But Americans might be surprised (some disappointed, some delighted) to find that a second President Clinton wouldn’t be much like the first one. The second President Clinton would have no time for frippery or flirting, and no patience for either. The second President Clinton would have the discipline to begin meetings on time, end them on schedule, and make sure that no important decision was made with a box of cold pizza open on the table. That would be an improvement.

Hidden emotion

I have no doubt that Hillary Clinton has strong emotions – the fact that her daughter is so poised and accomplished is a tribute to both of her parents, and there is no such thing as parenting without strong emotions. But no one has seen strong, authentic emotions from Sen. Clinton in a long time, at least not since her father died.

Part of presidential leadership is to feel the nation’s pain, and if it took a Bill Clinton phrase to express that thought, then that is proof enough that he mastered that part of the job.

Most voters aren’t worrying right now about discipline and emotion and character. They’re worrying about Iraq and the stock market and maybe abortion. But make no mistake. Their critics may refer to them, derisively, as “the Clintons,” but they are not the same person, and they wouldn’t be the same president.