The giant sitting on Hillary's shoulder
Many believe Bill will continue to be a major political advisor if his wife wins
Bill Clinton will have to carve out a new role if he enters the White House as the spouse of President Hillary.
Already, there has been speculation about what he will be called if she wins the election in November.
The most likely official title is "First Gentleman", which is already used for State governors. But, somehow, that seems unlikely to stick to Bill. On the campaign trail, the Clintons have joked that the title could be "First Dude" or "First Mate".
Since he left the White House, President Clinton has had his health problems. The man who used to devour burgers, fries and barbecued ribs has lost weight since he had quadruple-bypass surgery, and is now a vegan.
The 69-year-old cuts a frailer figure with his thin frame and flossy white hair.
An article in New York magazine estimated that he has earned a staggering $132m in speaking fees alone since he left the White House, and he remains a popular figure.
On the campaign trail, Bill's role is to bring some of the warmth and panache that his wife sometimes lacks - without overshadowing her own performances.
He gives few interviews, so as not to attract too much attention, and many of his appearances are at private fundraisers.
In his speeches, Bill tries to win over the white working-class, who have been veering towards Donald Trump rather than Hillary. She tends to have greater support among black voters, Hispanics, women, and those from wealthy and well-educated backgrounds.
Clinton is heavily involved behind the scenes in the campaign.
Although they often campaign separately, the Clintons are in constant phone contact.
He is said to keep a close eye on electoral strategy and polling data - regularly checking drafts of her speeches. Sometimes, he tries out particular lines and themes at his own events before they are incorporated into his wife's campaign.
Close observers of the Clintons believe he will continue to be a significant political advisor if Hillary wins the presidency. But this role may not be a public one.
Unlike previous First Spouses, Bill will certainly not be devoting time to picking out china, approving dinner menus and supervising floral arrangements.
Like previous First Spouses, he may pick up on some big themes without getting publicly involved in day-to-day politics.
Lady Bird Johnson, wife of Lyndon Johnson, championed environmental causes; Barbara Bush pushed literacy programmes; Hillary herself campaigned on healthcare; and Michelle Obama campaigns against childhood obesity.
While Bill remains popular, Donald Trump still sees him as a weak spot in the Clinton campaign.
The billionaire has already used Bill as a useful target to deflect any criticisms of his own treatment of women.
"She's married to a man who was the worst abuser of women in the history of politics," Trump has said of his opponent.
"Hillary was an enabler and she treated these women horribly. Just remember this. Some of these women were destroyed, not by him, but by the way that Hillary Clinton treated them after everything went down."
When colourful First Lady Betty Ford entered the White House in 1974, she was told she was "constitutionally required to be perfect". Given his past record, that will hardly be expected of Bill.
The public seems to have warmed to the idea of First Gentleman Bill, and "Bill Clinton for First Lady" badges and sweatshirts have appeared on sale.
The media mogul Tina Brown has no doubt that Bill will be an asset. "No president has had a more charismatic spouse than Bill in their armoury of surrogates," Brown has said in an interview. "The Big Dog will be envoyed into trouble spots as her advance charm offensive."