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Former Republican presidential nominee Mitt Romney told close supporters on a conference call on Friday that he will not run again in 2016, saying that he wants to allow other leaders in the GOP to step forward.

"After putting considerable thought into making another run for president, I've decided it is best to give other leaders in the Party the opportunity to become our next nominee," Romney said in remarks posted on radio host Hugh Hewitt's website, which participants on the call confirmed to NBC News as accurate.

Romney told backers he believed that he could have won the nomination but "it would have been difficult test and a hard fight."

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"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," he added.

Former Florida governor Jeb Bush, widely viewed as the new favorite of establishment Republicans, called Romney a "patriot" in a statement after news of the decision broke.

"Though I’m sure today’s decision was not easy, I know that Mitt Romney will never stop advocating for renewing America’s promise through upward mobility, encouraging free enterprise and strengthening our national defense," Bush said. "Mitt is a patriot and I join many in hoping his days of serving our nation and our party are not over."

Other potential candidates - including Marco Rubio and Rand Paul - also weighed in to praise Romney's leadership.

Some top Republicans had recently expressed skepticism about another presidential bid by the former Massachusetts governor, who failed to oust incumbent President Barack Obama in 2012. He was widely believed to have stepped away from presidential politics until early this year, when he told a meeting of donors that he was mulling another run.

Romney had also been contending with high-profile defections of supporters to Jeb Bush's camp. The latest came Thursday, when Romney's two-time Iowa guru, operative David Kochel, signed on to be Bush's national campaign manager if Bush pursues a run.

— NBC's Carrie Dann contributed to this report.