Martin Van Buren had Andrew Jackson. William Howard Taft had Theodore Roosevelt. George H.W. Bush had Ronald Reagan. George W. Bush had his father. Hillary Rodham Clinton has Bill Clinton.

Van Buren, Taft, Roosevelt and the two Bushes had the indispensable political assistance of powerful patron presidents, a formula that often works in American politics. But these presidential advocates were mentors and, in one case, a father as well. Bill Clinton is the last Democratic president, a wildly popular figure in his own party. And he is also Hillary Clinton’s husband.

Ultimate political weapon

But has there ever been a political arsenal with a weapon quite like Bill Clinton? Bill Clinton may not be the best husband alive, but he surely is the best political campaigner alive, the best political strategist alive, the best political symbol alive.

So the news that the Hillary Clinton presidential campaign is going to enlist the help and talents of Bill Clinton is no small thing. It is a bigger thing than Taft campaigning as TR’s handpicked successor, which helped Taft in 1908 but which turned sour once TR came to believe Taft had abandoned the Roosevelt approach in the White House, “completely (twisting) round the policies I advocated and acted upon.” By 1912, when Roosevelt ran against his onetime protege in one of the most bitter but colorful American elections ever, he would speak of “Taft’s muddleheaded inability.”

Bill Clinton’s likely involvement in the 2008 election is bigger, too, than Reagan campaigning for Vice President Bush, a relationship that helped Bush win the election but which had its undertones of resentment as well. In his inaugural address the new president spoke of an administration that would be kinder and gentler, prompting Nancy Reagan to wonder: kinder and gentler than whom?

And yet any precedents may be irrelevant. The Clintons are a partnership, to be sure, but above all they are a marriage. The skeptics, cynics and voyeurs can say what they wish, but even a marriage of convenience is a marriage, requiring gestures of respect and boundaries of action far different from those in even the closest political partnership.

In short, Bill and Hillary in 2008 are different from TR and Will in 1908.

Strengths, weaknesses

What does Mr. Clinton bring to the table? Not much in Iowa, oddly enough; in his 1992 race, the presence of Iowa Sen. Tom Harkin in the Democratic field rendered the state’s precinct caucuses all but meaningless, and Mr. Clinton spent precious little time there then or in 1996, when he faced no opposition in the caucuses. He does bring a lot in New Hampshire, where he had his first brush with political death at a paintbrush factory in Claremont when allegations broke that he had had an affair with Gennifer Flowers, a Little Rock television personality. He then campaigned like mad in New Hampshire, came in second, and painted himself as the “comeback kid,” one of the neatest bits of political packaging in modern political history.

But his real value is that there is nobody in Democratic circles who won’t take a call from Bill Clinton, or who won’t put his message slip at the top of the pile.

The lower President Bush sinks in the polls, and the more the Republican presidential candidates seek to establish their distance from the president, the more important an asset is Mr. Clinton. He’s the president people liked.

Now the dangers. Mr. Clinton was a rogue – a player, in the argot of today – and the Clinton years, though a period of robust economic growth, were not exactly a day at the beach. There was arguably more administration incompetence in the Clinton years than in the Bush years (though the Iraq war alone may trump the cumulative effect of Lani Guinier, the parade of attorney general nominees, the imbroglio over gays in the military, the missed opportunity on health-care overhaul and many, many more).

There was immense partisan anger. The impeachment ordeal was a disaster for all who touched or were touched by it. And lest we forget, the Clinton years are not remembered as a high-water mark for traditional family values.

Enhance, not diminish

The key is that in campaigning for his wife, whose ascension to the presidency would not surprise anyone who heard her speech at the Wellesley College commencement in 1969, Mr. Clinton must enhance and not diminish Sen. Clinton. Somehow Hillary Clinton has to reap the benefits but not relive the agony of the Clinton administration.

This is important to her politically, but it is vital to her psychologically. She cannot appear, or feel, like an appendage of the Bill Clinton mystique.

Her appeal must be for who she is, not for whom she married. Her candidacy, her presidency and her dignity depend on that more than on anything else.