WASHINGTON — He insists he could win, but Mitt Romney has stepped out of the 2016 presidential contest in favor of the “next generation of Republican leaders” following a three-week fact-finding effort that revealed significant resistance to a third campaign.

The 2012 Republican presidential nominee on Friday formally ended his flirtation with another White House bid and encouraged his supporters to seek another candidate from the crowded field of prospective GOP contenders. Aides said it was a deeply personal and even painful decision for Romney.

“I believe that one of our next generation of Republican leaders, one who may not be as well-known as I am today, one who has not yet taken their message across the country, one who is just getting started, may well emerge as being better able to defeat the Democrat nominee,” Romney told supporters on a conference call. “In fact, I expect and hope that to be the case.”

The remark was both a recognition of his own limitations and an indirect swipe at the man who created the urgency behind Romney’s brief flirtation with a third presidential campaign: former Florida Gov. Jeb Bush, the son and brother of former presidents who is speeding toward a campaign of his own.

Bush and New Jersey Gov. Chris Christie would have served as Romney’s most likely rivals for the support of the GOP establishment, and both men felt an immediate impact from Romney’s announcement. It sparked a rush of activity by Romney loyalists — operatives and donors alike — suddenly freed to support another White House hopeful as the crowded field begins to take shape.