House Speaker Paul Ryan tried Tuesday to put an end “once and for all” to the rampant speculation about his 2016 ambitions, ruling out a late-stage bid for the Republican presidential nomination and definitively declaring: “Count me out.”

The Wisconsin lawmaker and 2012 Republican vice presidential nominee made the announcement on Capitol Hill upon returning from the Middle East, where he said the speculation had followed him.

“I want to put that to rest once and for all,” Ryan said. “Let me be clear. I do not want, nor will I accept, the nomination for our party. ... Count me out.”

He also made clear that as speaker his role is to preside as chairman over the Republican convention in July, at which the GOP presidential nominee will be decided.

“My job is to ensure that there’s integrity in the process, that the rules are followed by the book,” Ryan said at the Republican National Committee headquarters.

Ryan’s name as a potential, last-minute candidate has circulated amid discontent in some parts of the so-called GOP establishment over the two leading candidates -- billionaire businessman Donald Trump and first-term Texas GOP Sen. Ted Cruz.

Despite Ryan's past insistence that he's not in the running, some have floated him as a potential alternative candidate in the event of a contested convention in Cleveland.

The potential for a contested convention has only increased as Trump struggles to reach the 1,237 delegates needed to secure the nomination. 

The speaker’s office stoked speculation in recent weeks with a series of sophisticated videos that appear to show Ryan on the so-called "big stage" including one shot from behind Ryan with his arms extended and showing an enormous crowd and a phalanx of TV cameras and still photographers.

Ryan has said 'no' before to a big party leadership job, only to reverse course.

When House Speaker John Boehner retired and House Majority Leader Kevin McCarthy, R-Calif., abruptly withdrew from the speaker’s race late last year, Ryan said, “While I am grateful for the encouragement I’ve received, I will not be a candidate.”

However, he agreed weeks later to take the post.

On Tuesday, he told reporters that trying to make that comparison was like "comparing apples to oranges."

Ryan also waded into the potentially messy politics of a contested convention, in which essentially anybody could win the nomination if neither Trump nor Cruz or Ohio Gov. John Kasich get the support of 1,237 delegates on the first round of balloting.

“Let me speak directly to the delegates on this: If no candidate has a majority on the first ballot, I believe you should only choose a person who actually participated in the primary,” he said. “If you want to be the nominee -- to be the president -- you should actually run for it.”

Nevertheless, Ryan vowed to remain relevant in the national debate that will help decide who becomes the next president, including presenting ways to improve free enterprise and the tax code and secure the U.S. border.  

“This job provides a platform to communicate a conservative vision for our country,” he said. “This is a critical role that has to be played, and I am in a position to play it.”

Fox News' Chad Pergram contributed to this report.