Hillary Clinton unleashes the big dog - but will Bill help or hinder her run at the White House?

“If you can’t run with the big dogs, then stay on the porch,” the old Southernism runs. But now the biggest dog of all is about to be let off the porch for the 2016 US presidential campaign. Starting on Monday in New Hampshire, Bill Clinton will be on the stump for his wife in the key early primary states. There’s just one problem. Courtesy of Hillary, he’s already been dragged into a brawl with – who else? – Donald Trump.

Bill has always been her not-so-secret weapon. True, he wasn’t much of an asset back in 2008, when some cack-handed remarks he made about Barack Obama only strengthened her opponent’s hand. But this time there’s no Obama to worry about, just the less charismatic figure of Bernie Sanders, whose scant hopes of winning the Democratic nomination reside in upset wins in those first states.

Whatever else, Bill Clinton remains the big dog of American public life. He’s not just the most gifted, but the most popular politician of his era – much more popular, polls show, than his wife, Obama, or any of the Republicans now running for the White House. And that’s not just because Americans tend to prefer their presidents retired (even George W Bush is enjoying a measure of rehabilitation).

Clinton is white-haired now, and noticeably thinner than in his pomp. After a heart scare a decade ago, he takes much better care of himself than in his pizza-guzzling youth. But he can still connect with an audience as no other, reminding voters of a happier and more prosperous time, that “end-of-history” period between the end of the Cold War and the ghastly wake-up call of 9/11, when American power was unchallenged and even the federal budget was briefly in surplus.

Which leads to another Southern idiom of which Clinton is fond when he looks back on his presidency: “If you see a turtle on a fence post, it didn’t get there by accident.” The reptile in question is the economic boom of the Clinton years, and it didn’t happen by accident – but as a result of the sagacity of the 42nd US President. Add to this his unmatched ability as a fundraiser, and you can see why Hillary wants him in the thick of the fray.

There are one or two problems, however. It’s been almost 15 years since the Clintons left the White House. Like it or not, Bill represents the past, his every utterance a reminder that the couple have been centre-stage in public life for a quarter of a century. Yes, they have vast experience, but US presidential elections are almost always about the future. Hillary’s toughest opponent in November would probably be the Florida senator Marco Rubio, only 44 and seriously short of experience, but able to present himself as a symbol of generational change, just as Bill Clinton did when he beat the elder Bush in 1992.

And then there’s another problem. As everyone knows, the big dog in those days had a habit of slipping off the porch for certain extra-curricular activities which many Americans do not remember so fondly – the “bimbo eruptions” featuring Gennifer Flowers, Paula Jones and, of course, the biggest eruption of them all, Monica Lewinsky. And now they are about to be revisited once more, thanks to his wife and Donald Trump.

Once upon a time the Clintons and Trump were good friends, at least by the mutually congratulatory standards of the celebrity universe that they inhabited. “I think he’s a terrific guy,” Trump said at the height of the Lewinsky scandal. He and Bill were golfing buddies, while Trump donated money to the Clinton Foundation, to Hillary’s Senate campaign, and to her 2008 presidential bid. The Clintons attended Trump’s third wedding, to the Slovenian model Melania Knauss, in 2005 (though Trump wasn’t invited to Chelsea Clinton’s 2010 wedding to Marc Mezvinsky). But such comity is history now.

The trouble started when Hillary spoke of Trump’s “penchant for sexism” – which is putting it mildly, given his crude remarks about the Fox News anchor Megyn Kelly, his Republican presidential rival Carly Fiorina and Hillary’s “disgusting” visits to the bathroom during breaks in a recent Democratic candidates debate.

But dish it out to Trump at your peril. Where others see merely an insult, he sees an opening. Sexist, who’s sexist? “If Hillary thinks she can unleash her husband while playing the women’s card against me, she’s wrong,” Trump tweeted. “Her husband’s one of the great abusers of the world, give me a break.” He was utterly confident. “We’ll go after the ex-president,” he told reporters; “it’ll come out well for us.”

Maybe it will. You can see this as a nifty move by Trump, taking aim not at his primary rivals, but at the Democrat he would almost certainly face in the election. In doing so, he bolsters his reputation as a man who bellows forth what conventional politicians dare only hint at. That’s a big reason why fans love him. And maybe it will prompt some second thoughts about Hillary as a champion of women’s rights by reminding us of the women who fell victim to Bill’s zipper problem.

Equally though, the gambit could backfire. If anyone in US politics can handle this sort of attack, it’s the Clintons. She prevails by staying relentlessly on message, while no one beats Bill at skewering opponents on a jokey “aw-shucks” stiletto. And Republicans, fixated by a belief that the Clintons are the root of all evil, have a terrible propensity to overreach.

It happened when they impeached Bill over the Lewinsky affair, and only made the country feel sorry for him. It happened just three months ago, when a Republican Congressional Committee hauled Hillary over the coals for the umpteenth time over the Benghazi affair. It produced not an iota of new information. During eight hours of questioning she ran rings around her interrogators, who came off as a bunch of incompetent bullies.

Will Donald Trump have better luck? Not if the big dog has anything to do with it.