Democratic presidential candidate Hillary Clinton and her husband, former President Bill Clinton, share a moment onstage during the Central Iowa Democrats fall barbecue, Nov. 15, 2015, at Hansen Agriculture Student Learning Center of Iowa State University in Ames, Iowa. Alex Wong/Getty Images

As Bill Clinton prepares to hit the campaign trail in support of Hillary Clinton’s 2016 White House bid, Donald Trump has been calling attention to the former president's past marital infidelity. But despite attacks that may conjure snickers and memories of impeachment in some circles, the 42nd president's star power is unlikely to be hurt by Trump. His involvement in Hillary Clinton's presidential campaign is already more robust — and more nuanced — than it was in 2008, when his tendency for unpredictable comments during public appearances landed him on the sidelines. And with his first public, solo campaign events coming next week in early January, the former secretary of state is counting on her husband to help excite voters ahead of the first nominating contests in Iowa and New Hampshire.

“There’s always that sense of ‘what’s around the corner?’ You don’t know what’s coming. That’s part of the excitement of Bill Clinton,” said Gil Troy, a history professor at McGill University in Canada who recently wrote a book called “The Age of Clinton: America in the 1990s” about the couple’s political careers. “He’s that boyfriend, that bad-boy boyfriend of the American people. The charm and unpredictability and sexiness are part of Bill Clinton.”

The feud between Trump and Hillary Clinton has been heating up in recent weeks as both candidates look toward a general election in which they could easily face one another. But the past few days have seen Trump turn toward the 42nd president as his latest punching bag. After Hillary Clinton said last week that the New York billionaire had a “penchant for sexism,” Trump responded by accusing her husband of having a “terrible record of women abuse” on Twitter.

Bill Clinton’s extramarital activities are no secret. Most famous, of course, is his affair with White House intern Monica Lewinsky, which led to his impeachment, and Paula Jones, a former Arkansas state employee, who filed a sexual harassment suit against him during his presidency. Former campaign volunteer Juanita Broaddrick and former White House volunteer Kathleen Willey have both accused Bill Clinton of sexually assaulting them, and other women have made allegations about inappropriate advances or sexual harassment, but the former president has never been charged in any of these situations.

Bill Clinton is widely considered one of the most popular presidents in recent history and has managed to maintain that likability in the years since he has left office. This past May, the most recent time Gallup measured how favorable Hillary Clinton and her husband are, he scored nine points above the former secretary of state, with 59 percent of respondents viewing him favorably to her 50 percent.

“Had [Bill] Clinton resigned when over 100 papers called for him to resign, his presidency would have been defined by that,” Troy said. “Bill Clinton wrote the final chapter of his presidency so that he ended with higher popularity than Ronald Reagan, with peace, prosperity — so that’s the legend of Bill Clinton.”

President Bill Clinton - Approval Ratings | InsideGov

This enduring stardom, particularly among Democrats, means that the former president often draws large crowds and is now being referred to by his wife as her “not-so-secret weapon.” But in 2008, when Hillary Clinton was running against Barack Obama, her husband was viewed as a distraction who could overshadow the sometimes-wooden candidate, and was given specific areas of involvement to occupy him, Politico reported.

His folksy style also led him to go off-script at campaign events, a pattern that got him into trouble several times and culminated in his damaging remarks about Obama after the Clintons badly lost the South Carolina primary. While explaining Hillary Clinton’s failing numbers in the early primary state, her husband reminded reporters that Jesse Jackson won the South Carolina primaries in his unsuccessful runs for president in the 1980s — comments that were interpreted as dismissing Obama’s success based on his race.

Although Bill Clinton denied that interpretation, Obama’s campaign took up the cause and it was a moment of significant controversy. He also made a mistake when he revived an embarrassingly false account Hillary Clinton told about her visit to Bosnia in the 1990s, which she said had been under “sniper fire.” In an attempt to defend his wife, the former president brought attention back to the gaffe and frustrated Hillary Clinton’s campaign.

After the incident in South Carolina, Hillary Clinton largely kept her husband out of the public eye, meaning his contributions were concentrated in the months of January through March that year. But now, with the 2016 campaign heating up long before the primary elections even start, the former president has already appeared to introduce his wife and pop star Katy Perry at the important Iowa Jefferson-Jackson Dinner in October and has been attending fundraisers over the past month.

A big concern about involving Bill Clinton on the campaign trail has always been that his charm can easily outshine the former secretary of state and serve to highlight her political awkwardness.

“In 2008, we saw some conflict there. No one campaigned harder for Hillary Clinton than Bill, but there were times that it didn’t help,” said Meena Bose, the director of Hofstra University’s Center for the Study of the American Presidency in New York. “In 2016, Bill Clinton’s role will likely be one of rousing the crowds but staying more in the background…Without a major opponent like Obama, I doubt we’ll see the same kind of issue.”

A lot has changed since the Clintons' last national campaign. While Hillary Clinton served as secretary of state under Obama, her husband’s popularity rose to its highest point, and her front-runner status is allowing her to more confidently use her husband’s political expertise this time around.

“I think 2008 was a very jarring and searing experience for both of them. There was a lot of fury because they felt the Democratic party was their personal property,” Troy said. “I think they’ve learned how to keep him more under wraps and make sure it’s really her show, not his show.”

But 2016 brings its own political environment as well, and one that may be less friendly to the old persona of a bad-boy former president with dalliances outside his marriage. Hillary Clinton has already been asked by a voter in New Hampshire if her support for believing sexual assault survivors should apply to the women who have made allegations against her husband. At the time, she replied that “everyone should be believed at first until they are disbelieved based on evidence” and smoothly moved on.

So far, Trump's criticisms on this topic seem unlikely to stick with voters who like the former president and are excited about the prospect of Hillary as the first female president. But if Trump were to get more personal and more detailed with his attacks, he could try to tap into a growing national sentiment that has condemned celebrities like Bill Cosby, R. Kelly and Anthony Weiner for sexually inappropriate conduct in recent years. Still, experts say that for some voters, the 42nd president simply can't be touched.

“In a modern America, in a post-sexual revolution America…that kind of sexual promiscuity isn’t as scandalous and problematic as it once was,” Troy said. “The American people feel he paid his dues — he was punished; he was impeached. There’s a certain kind of unfairness, no matter how outrageous his behavior was, no matter how borderline legal, he was so successful at what I call the Clinton jiu jitsu, at blaming it on the Republicans and making it a partisan issue, that he came out well.”

Correction, Dec. 30, 2015, 8:50 a.m. EST: A previous version of this story erroneously identified Bill Clinton as the 41st president of the United States. In actuality, he was the 42nd president.