Where does Bill fit in presidential puzzle?

Hillary Clinton has put an end to speculation about one question: Yes, she is running for president. But that leaves another: What role has she in mind for her husband?

The former president is seen as a formidable campaigner and potential asset to his wife's operation. But would he outshine her? And should his own baggage prove too much to carry?

While many voters retain fond memories of Bill Clinton's 1990s terms as an era of economic strength and international prestige for the United States, next year's election will turn on visions of the future.

Mrs Clinton's Republican opponents, in particular the youthful 43-year-old Senator Marco Rubio, are striving to portray her politics as a tired throwback to the baby-boomer generation.

If Mrs Clinton wins, she will be 69 when she takes office, the same age as Ronald Reagan, and more than two decades older than Mr Clinton or Barack Obama were at their inaugurations. Having a beaming Mr Clinton standing beside her can only remind voters that she has been a fixture of US public life for 35 years, a respected figure but not a blast of fresh air.

Mr Obama beat Mrs Clinton in the 2008 Democratic primary running as a fresh-faced candidate for hope and change - and Mr Clinton's maladroit interventions scarcely helped his wife.

Many of the black voters who flocked to Mr Obama's standard were offended by Mr Clinton's dismissal of the challenger's success in South Carolina, with its large African-American electorate.

Out of office, the Clinton family foundation is an asset. The multi-million-dollar charity gives the Clintons and daughter Chelsea a statesman's visibility on the global circuit. But Mrs Clinton stood down from the board on Sunday after she launched her run - the grants the fund received from foreign governments carry the whiff of a conflict of interest.

Both Clintons are used to probing - even harsh - press coverage, but the nature of the upcoming Democratic primary race threatens them with a new level of scrutiny. In the absence of a credible challenger from within her own party, reporters covering her race will have one eye on Mr Clinton and another on a barrage of attacks from the Republican camp.

Mrs Clinton is no longer defined as a former first lady. Since the couple left the White House at the end of Mr Clinton's second term, she has become a leading senator and diplomat in her own right. And she has bristled when Mr Clinton's sometimes clumsy interventions have overshadowed her own efforts, or cast her as in need of protection.

Last year, when Mr Clinton came to her defence over some ill-judged remarks about her finances, Mrs Clinton declared: "My husband was very sweet today, but I don't need anybody to defend my record. I think my record speaks for itself."

Mr Clinton, while clearly keen to play a role, is aware of the unique threats he poses to his wife's efforts. When asked what position he saw for himself in the campaign, he laughed and affected modesty: "I'm a foot soldier in an army. I will do what I am instructed to do."

No one in Washington really believes that, though. Mr Clinton is a political animal and will find it hard to stand by as his wife goes through it alone against their shared opponents.

According to a recent poll by NBC News and the Wall Street Journal, 56 per cent of US voters have a positive opinion of the president who was once impeached for lying about an affair. This compares with only 44 per cent for his wronged wife, despite her Democratic frontrunner status. Thirty-six per cent of voters have a negative opinion of her, only 26 per cent of him.

Mr Clinton's approval ratings also outstrip Mr Obama's and, by endorsing his wife's former opponent, boosted the President's 2012 re-election drive and helped mend fences with black voters. His rapturously received speech at Mr Obama's nomination convention earned Mr Clinton the White House's thanks and the tongue-in-cheek title of "Secretary for Explaining Stuff".

But could Mr Clinton also be his wife's saviour if her campaign runs into trouble? Brendan Nyhan, professor of political science at Dartmouth College, thinks not.

"Bill Clinton is obviously a very gifted politician, but he campaigned for plenty of candidates who could not survive when conditions were unfavourable, and he could not save Hillary in 2008," he noted.

In the 600 days remaining before America goes to the polls on Nov 8, 2016, Mr Clinton can play to his strengths as a party fund-raiser and backroom adviser with a sharp political mind. But it will be for Mrs Clinton to find her own magic on the stump.

"There are no magic words he can say if the conditions are bad, and he is vulnerable to becoming the story himself when he tries to defend her," Prof Nyhan warned.