Mitt Romney and Foster Friess, a wealthy donor to conservative causes, were walking out of an event together a few months ago when Mr. Friess broke the news: After backing Mr. Romney for president four years ago, he was getting behind Rick Santorum this time around.

“He couldn’t quite figure out why Rick was even bothering to go through the effort,” Mr. Friess recalled in an interview on Wednesday. “I mean, I don’t mean to fault him for saying, ‘Why take Rick seriously?’ Nobody took Rick seriously.”

Many more Republicans are taking Mr. Santorum seriously now, thanks to his victories in Minnesota, Missouri and Colorado on Tuesday — and perhaps none more than Mr. Romney, for whom Mr. Santorum’s unexpected rise poses another threat from the right.

Few people played a more pivotal role in Tuesday’s turn of events than Mr. Friess. An investor who made millions in mutual funds and now lives in Wyoming, he is the chief backer of a “super PAC” that has helped keep Mr. Santorum’s candidacy alive by running television advertisements on his behalf.