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Adding another unanticipated sidebar to this topsy-turvy election, Kenneth Starr has lavished praise on Bill Clinton, citing his "genuine empathy for human beings", calling him "the most gifted politician of the baby boomer generation" and commending his post-presidential philanthropy, which he noted was Carteresque in its benevolence.

Starr, a former independent counsel, was the author of what's probably the most expensive piece of pornography ever published, the Starr Report which chronicled, in graphic sexual detail, Bill Clinton's affair with a 21-year-old White House intern, Monica Lewinsky.

The one-time character assassin has become a character witness.

Bill Clinton, despite being impeached for lying under oath about that affair, left office with the highest approval rating - 66% - of any departing president.

He was also credited with balancing the federal budget and reviving the American economy from its early-Nineties slump.

Since then, as Starr noted, he has followed a redemptive path. So he should be a prime asset to his wife Hillary as she seeks to become the first ever first lady to move from the East Wing of the White House to the West.

The irony of Starr's kind words is that they have come at a time when Hillary, among other woes, has a Bill problem. The man who could well become the "First Dude" - Hillary's words, not mine - is proving to be something of a liability.

Part of it stems, of course, from what eventually became the focus of Kenneth Starr's inquiry, Bill Clinton's womanising.

Image copyright Getty Images Image caption Photos of Lewinsky with Clinton were submitted to the Starr investigation

Image copyright Getty Images Image caption Starr's endorsement came as some surprise, given he was one tormenter-in-chief

It blunts her attacks on Donald Trump's sexism and misogyny which, in a contest where more women will vote than men, should have been her ace card.

The billionaire, in a jujutsu-like move, has already launched an attack ad featuring video of Kathleen Willey, who accused Bill Clinton of sexual assault, and Juanita Broaddrick, who accused him of rape.

"DIRTY BILLS," read the front page of the New York Post the following day, conflating Trump's attack ad with the allegations against Bill Cosby.

Nor is Trump relying on guilt by association. He has already called Hillary an "enabler" of her husband's fidelity. "She would go after these women and destroy their lives," he has claimed.

Image copyright Getty Images Image caption The Clintons presented a united front at the height of the scandal

Like many of Trump's attacks, that may strike many voters as shrill and overstated, it also contains a kernel of truth.

The White House did attempt to trash Monica Lewinsky when the scandal first erupted in 1998, and Hillary Clinton, during her husband's long phase of public denial, described it as being the product of a "vast right-wing conspiracy", absolving him of blame.

Hillary as loyal "stand-by-your-man" wife does not marry that well with Hillary as feminist trailblazer, the image she is trying to project.

My sense, having watched the Clintons fairly closely for over 20 years, is that they have a loving marriage and deep friendship. An animated conversation that started at Yale Law School in the early Seventies continues for both of them to fascinate and enthral.

But many critics of the Clintons believe it's a transactional partnership, a marriage of political convenience. To some, her loyalty during the most troubled phase of the Clinton presidency reinforces the sense that she's a cynical political operator, willing to do anything to accrue power.

Image copyright Getty Images Image caption Few relationships in US public life have been so scrutinised

The Bernie problem, her difficulty in seeing off her Democratic rival, is also partly a Bill problem. The unexpected success of the Vermont Senator is explained not only by an aversion to Hillary but also a rejection of Bill.

Sanders supporters are railing against his political and policy legacy. Bill Clinton's pursuit of a "Third Way" politics was designed to make the Democrats more electable and to end the party's losing streak in presidential politics.

Up until his victory in 1992, they had only won one of the previous six elections. But this shift to the middle ground, and the centrist policies that accompanied it, alienated many on the Democrat left. Withholding support from Hillary Clinton and bestowing it instead upon Bernie Sanders is a form of revenge.

Whether its financial deregulation, the welfare bill that Clinton passed with the Republican-controlled Congress, criminal justice reforms which have contributed to higher levels of black incarceration, the North American Free Trade Agreement, or Nafta for short, there's been an angry backlash against what Clinton once trumpeted as some of his greatest domestic accomplishments.

Image copyright Getty Images Image caption The trade agreement used to be a badge of honour for Bill Clinton

This has made her vulnerable to a challenge from the left, as Sanders has shown. But it also makes her vulnerable to a challenge from the right, because of Nafta.

Blue-collar voters, who might ordinarily be expected to vote Democrat, stand to applaud when Donald Trump promises to renegotiate trade deals which have exported American jobs abroad.

Hillary Clinton hinted recently that Bill would become her jobs tsar and focus his energies on reviving American manufacturing.

She clearly hopes that he can appeal again to blue-collar whites, who warmed to him as a candidate and president. But that bond has been severely eroded, if not severed completely, because he was the president who negotiated Nafta.

Image copyright Getty Images Image caption Trump has railed against trade deals and won huge blue-collar support

Public perceptions about Bill Clinton have also changed, in a way which exposes the rupture between the Democratic establishment and the Democratic grassroots.

When he first appeared on the national scene, he could plausibly cast himself as the boy from Hope, his small-town Arkansas birthplace.

Now, after all the millions he has earned on the international speaking circuit, he comes across not just as a limousine liberal but a Lear Jet liberal. Standing at the head of a metropolitan progressive elite, he does not have the same common touch of old.

The Clintons, after all these years as the dominant power couple in the Democratic establishment, also project an imperious sense of entitlement. It explains why Hillary Clinton's email scandal cuts so deep. It reinforces the widespread view that the couple believe they are not bound by normal rules.

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Those of us who have watched him on the campaign trail have also been surprised at the 69-year-old's comparative lack of energy. The magnetism and charisma for which he is famed simply is not there.

At a rally in New Hampshire, I happened to be standing next to Clinton's biographer David Maraniss, whose real-time Twitter feed made for fascinating reading.

"When BC was introduced and stood on stage w/Chelsea, he showed nothing on his face, mouth agape, eyes seemingly blank... just frail, like he had to conserve every ounce of energy. No gleam in his eyes, no electricity, muted…. He lit up only when Chelsea talked about him. Then when it was his turn to talk a little bit of his old self came back, but not much."

Once the most powerful energy force in any room, Bill Clinton is now only an ambient presence.

Image copyright Getty Images Image caption Bill Clinton is still campaigning but perhaps not with the same vigour of old

The former president is not without his fans. There are thousands who still cheer this self-proclaimed architect of the bridge to the 21st Century. What's also been striking is how African-American voters have returned to the Clinton fold, after backing Barack Obama in 2008, despite protesters from the Black Lives Matters campaign targeting him on the stump.

However, the former first couple's hopes that wistful memories of the Clinton administration would lead to a Clinton restoration have surely dwindled.

Nostalgia is something that can give a departing president a warm glow, as Barack Obama is presently discovering, but hard for a presidential aspirant to harness, especially when so much of the country is angrily demanding change.