(AP Photo/J. Scott Applewhite, File)

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Rep. Paul Ryan (Wis.), the 2012 Republican nominee for vice president, said Monday that he will not run for president, ending speculation about one of the highest-profile prospects for the 2016 campaign.

"I have decided that I am not going to run," Ryan said in an interview with NBC News. He added that he is "at peace" with the decision he made "weeks ago."

Ryan had long been rumored as a potential White House hopeful in the next election. Viewed as a rising star in the party who benefited from experiencing the rigors of a national campaign in 2012, the Wisconsin Republican was seen by many party officials as a top contender-in-waiting.

But unlike some of his peers, Ryan never appeared to be ramping up for a run, signaling instead that he was content to pour himself into making fiscal policy in the Republican-controlled House during the next two years. Ryan is the chairman of the powerful Ways and Means Committee, which is tasked with tax-writing.

In his interview with NBC, Ryan confirmed that contentment.

"It is amazing the amount of encouragement I have gotten from people, from friends and supporters," he said, adding, "but I feel like I am in a position to make a big difference where I am, and I want to do that."

Ryan's endorsement in the Republican primary will be a highly sought-after stamp of approval. But he said Monday that he is not ready to back a specific candidate yet. The 2016 GOP field is beginning to take shape, as several potential candidates are racing to secure the donors and senior staff required to wage a national campaign.

Former Florida governor Jeb Bush is exploring a bid and has taken steps toward becoming an official candidate in recent weeks. The 2012 nominee, Mitt Romney, declared serious interest last week in running again.

Ryan's home state governor, Scott Walker, is another possibility. So are several of Ryan's colleagues in Congress, including Sens. Rand Paul (Ky.), Ted Cruz (Tex.) and Marco Rubio (Fla.).

Govs. Chris Christie (N.J.), Bobby Jindal (La.) and Mike Pence (Ind.) are also seen as possible candidates.

On the Democratic side, former secretary of state Hillary Clinton is seen as the presumptive frontrunner. Clinton has not yet said whether she will run.

Ryan, a disciple of the late congressman and U.S. housing secretary Jack Kemp, has served in the House since 1999. In recent years, he has become a leading Republican figure on fiscal issues, crafting budget blueprints that would revamp entitlement spending, making a him a hero to the conservative base but a villain to liberal activists.