A Mississippi Republican who turned down a reporter's request for a campaign ride-along says he was following the "Billy Graham rule.

Robert Foster told Mississippi Today reporter Larrison Campbell that he would only agree to the 15-hour shared trip if she brought a male colleague along for the ride. 

She says she was told by Foster's campaign director, Colton Robison, that "the optics of the candidate with a woman, even a working reporter, could be used in a smear campaign to insinuate an extramarital affair."

Foster has been on the campaign trail ahead of next month's primary election. Following a backlash, he said his decision not to ride alone with Campbell was down to his commitment to his wife and the "Billy Graham rule" - a practice named after the late evangelist who, out of principle, avoided being alone with any woman he was not married to.

The 'Billy Graham rule' is popular among male evangelicals and conservatives, including Vice President Mike Pence, leading to it also being known as the 'Mike Pence rule'.

Some have accused Foster of sexism.  Washington Post columnist Monica Hesse claimed that the 'Billy Graham rule' demeans women. 

"They just presume that your marriage vows are so flimsy that you can't be trusted to uphold them unless a babysitter monitors you," she wrote.

"It's rather like a thief sanctimoniously announcing that he brings a parole officer every time he goes to the bank to make sure he doesn't rob it. Good for you, dude, for knowing your own limitations — but it doesn't make you better than the rest of us, who manage to regularly not steal things even when we're completely alone."

Foster defended his decision on Twitter, saying: "Before our decision to run, my wife and I made a commitment to follow the 'Billy Graham Rule,' which is to avoid any situation that may evoke suspicion or compromise of our marriage. 

"I am sorry Ms. Campbell doesn't share these views, but my decision was out of respect of my wife."

He was equally dismissive of the criticism voiced in the newspapers.

"Neither the New York Times or the Washington Post approve of my values. Good. That means I'm the right side of things," he tweeted.