Updated at 11:58 a.m.
Former Republican presidential nominee Mitt Romney isn't running for president in 2016, he told supporters in a statement.
"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," he said in the statement.
The former Massachusetts governor was the GOP nominee in 2012, but lost to President Obama in the general election. He also ran in 2008, but lost to eventual Republican nominee, Sen. John McCain of Arizona.
NPR's Tamara Keith tells our Newscast unit that "Romney had been floating trial balloons, giving speeches and talking to donors in recent weeks. He said on the call that he was convinced he could win the nomination again, but that stepping aside would be best for the Republican party and the nation."
Romney's decision still leaves a crowded Republican field. Although there have been no other major announcements, former Florida Gov. Jeb Bush is considering a run, as are Florida Sen. Marco Rubio and Kentucky Sen. Rand Paul.
Bush, in a posting on his Facebook page, said while today's decision was unlikely to have been easy for Romney, "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."
Paul, on Twitter, said that he deeply respects Romney's service.
On the Democratic side, former Secretary of State Hillary Clinton is widely expected to be the party's nominee.