This week, he appeared on the Late Show with David Letterman. Would he move back into 1600 Pennsylvania Avenue if Hillary won? "The chances are a 100 per cent I'll move back - if I'm asked."
And there's the rub. During Hillary's tenure as first lady, opponents accused her of meddling in affairs of state. How hands-on, or hands off, would Bill be?
If Hillary takes charge of America's nuclear strike codes, what will be Bill's role as first gentleman? A roving ambassadorship has been suggested. A cabinet post seems less likely.
And what should he do during Hillary's campaign. Stay out of sight and help with fundraising, strategy and tactics? Or engage with voters and help take Hillary's fight to Republicans?
Bill has sharp political instincts and - unlike Hillary, who values privacy - revels in partisan cut-and-thrust, and reaching out to voters.
As Hillary hits the campaign trail, Bill has vowed -- most recently last month to Town and Country magazine, which focused on the troubled Bill, Hillary and Chelsea Clinton Foundation - he will remain a "backstage adviser". Until, that is, "we get much, much closer to the election".
Last June, he told the Denver Post he was "a bit player and whatever she wants to do is fine by me".
Even though he has occupied himself with Clinton Foundation business, political animals have swallowed low-key Bill with a pinch of salt.
"Everyone chuckled," said the director of the Centre for Politics at the University of Virginia, Larry Sabato. "He doesn't have that kind of discipline. Stay out of it? He doesn't know how. He can't."
And Bill's missteps can chill Camp Clinton. During the 2008 White House race, Bill often introduced his wife on the hustings, and even campaigned for her independently. But he all but torpedoed her chances by denigrating Obama, provoking accusations of racism when he said the campaign run by Hillary's rival was "the biggest fairytale I've ever seen".
"He's an asset because he was a popular President, is well remembered and represents a time of peace and prosperity," said Sabato.
"He's a liability because he brings a lot of baggage -- personal scandals -- and, oddly, an inability to control what he says and how he says it."
But while Bill's missteps may hurt Hillary, Republican efforts to beat the bushes for new Clinton scandals are met with shrugs by Americans -- after all, Bill beat impeachment in the Monica Lewinsky affair.
More recently, Hillary's enemies have focused on possible financial impropriety. The latest missive, Clinton Cash: The Untold Story of How and Why Governments and Businesses Helped Make Bill and Hillary Rich, by conservative author Peter Schweizer, tries to smear Hillary by linking donations, lobbyists and her schedule as Secretary of State, implying policy could be bought. But, so far, no smoking gun has surfaced.
Critics have also blasted Bill's lucrative career as a public talker. He makes up to US$500,000 ($669,235) per speech and has made US$105 million in the past 12 years, including US$50 million during Hillary's time at State, when Bill's speeches had to be vetted for conflict of interest. Apparently, they were all cleared.
But Bill's insistence the US$50 million was needed to "pay the bills" suggested a surprising tin ear for popular sentiment; many Americans have done it hard in the global recession. Hillary seems similarly aggrieved, insisting the Clintons were "dead broke" when they left the White House.
As ex-Presidents are supported by federal largess this is risible.
Masters of 1990s triangulation politics, the Clintons straddled political wings to seize the centre. Bill passed the North American Free Trade Agreement and bent to corporate demands for draconian copyright legislation and less Wall St regulation.
Now the wind has shifted. Hillary has found it expedient to fence-sit on the Trans-Pacific Partnership as her party's left rallies behind Senator Elizabeth Warren, and rival presidential candidates Senator Bernie Saunders and Martin O'Malley, blasting the secret deal as anathema to middle-class voters.
The TPP is already proving politically toxic for Democrats. This week Obama was humiliated in the Senate, when Democrats defied his bid for fast track authority to pass it and a similar treaty with Europe.
A day later, however, Obama had struck a Senate compromise to expedite his trade agenda before it ignites the left in presidential primaries. Hillary must recalibrate her policies to capture the spirit of a new age that spawned the Occupy Movement, the backlash against government surveillance and a widespread sense that ordinary Americans are missing out.
Which means distancing herself from her husband's policies, a tricky challenge for a presidential candidate.
It remains to be seen if Bill will be an asset or an albatross for Hillary.
"Bill Clinton is like nuclear energy," Obama strategist David Axelrod told the Washington Post. "If you use it properly, it can be enormously helpful and proactive. If you misuse it, it can be catastrophic."
