In 1990, during a close and bitter congressional race, Mike Pence came under attack for using campaign funds to pay for personal expenses, including his mortgage and credit card bills.
The expenditures were not illegal at the time, but proved difficult to explain for a candidate who had railed about the outsized role of money in politics.
Mr Pence in turn blasted his opponent with attack ads, including one featuring a man in traditional Arab clothing who thanked Mr Pence's opponent in thickly accented English for policies that benefited Middle Eastern oil producers. The commercial was attacked by Arab-American groups, and the Indianapolis Star called it one of the two worst campaign commercials that year.
Mr Pence, who is now Indiana's governor and was selected this week to be Donald Trump's presidential running mate, lost his 1990 race, but what he regretted more than losing, he later said, was his decision to sling mud.
Donald Trump and Mike Pence following the VP announcement (Getty)
In the 25 years since that loss, Mr Pence's political allies and enemies alike say he has steered clear of personal attacks.
This could prove challenging when campaigning with Trump, who enthusiastically tears into his rivals.
Mr Pence's style is likely to cast him in a very different role from that of traditional vice presidential candidates, who often throw and take punches to allow presidential contenders to remain above the fray.
In the case of Mr Trump and Mr Pence, said Indianapolis University Professor Laura Merrifield Albright, the roles are likely to be reversed.
"It's a different dynamic. It's tough to imagine anyone out-trumping Trump," she said, noting that he is "willing to do his own dirty work".
Michael Totten, a retired architect who worked on Mr Pence's 1990 campaign, thinks the two candidates will complement one another. He dubbed Mr Pence "the perfect No. 2," a running mate who can "temper some of Trump's enthusiasm" and "be the calming voice".
After losing his 1990 race, Mr Pence apologised to his opponent for the oil advertisement.
He later told his colleague at a conservative think tank, William Styring, that he regretted the tone of the campaign. "I really screwed up on this. It's not me," Mr Styring recalled Mr Pence as saying.
In 1991, Mr Pence wrote an apologetic article for the think tank's policy journal, titled Confessions of a Negative Campaigner.
After the defeat, Mr Pence did not seek public office again for a decade. He worked at the Indiana Policy Review and hosted his own statewide conservative radio talk show, describing himself as "Rush Limbaugh on decaf". He also hosted a morning TV show in Indianapolis from 1995 to 1999.
In 2000, Mr Pence again ran for a US House of Representatives seat, this time successfully.
And he demonstrated that his 1990 mea culpa was more than just political theatre, observers of Indiana politics say. His opponents in campaigns after he re-entered politics describe him as extremely disciplined and cordial.
Melina Fox, Mr Pence's Democratic challenger in 2002, said that his statements in the race were "calculated and thought-out". Democrat Barry Welsh, who ran against him in 2006, 2008 and 2010, said Mr Pence was invariably "polite, professional and gentlemanly".
But some question how he will cope with being constantly challenged in a hard-fought presidential race.
"He seems uncomfortable with difficult questions and does not take counterpunches very well," said Rebecca Pearcey, who ran the losing campaign of Mr Pence's opponent in the 2012 race for Indiana governor, Democrat John Gregg. "He doesn't engage. He operates as though the opponent is not there."
It remains to be seen how aggressive Mr Pence will be in campaigning for Trump in coming months - and whether he will stick to the principles outlined in his 1991 article on negative campaigning.