Republican presidential hopeful Sen. Marco Rubio, R-Fla., speaks Wednesday before the Council on Foreign Relations in New York, where he laid out his "Rubio Doctrine." Mary Altaffer/AP hide caption

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Republican presidential hopeful Sen. Ted Cruz speaks at the Republican Leadership Summit Saturday in Nashua, N.H. Jim Cole/AP hide caption

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Senate Foreign Relations Committee member Sen. Marco Rubio, R-Fla., questions Secretary of State John Kerry on Capitol Hill last month. In an interview with NPR, Rubio reiterated his opposition to President Obama's dealings with Iran and Cuba. Pablo Martinez Monsivais/AP hide caption

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A protester in front of Sen. Marco Rubio's Doral, Fla., office in 2013 urges Rubio to stop opposing the inclusion of lesbian, gay, bisexual and transgender families in the Senate's immigration bill. Joe Raedle/Getty Images hide caption

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Marco Rubio celebrates onstage with his family in 2010 after winning his U.S. Senate seat in Florida when he was just 39 years old. Now, he's expected to embark on a run for president. Alan Diaz/AP hide caption

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Sen. Rand Paul, R-Ky., center, meets with members of the Londonderry Fish and Game Club in Litchfield, N.H., on Jan. 14. Paul was one of three GOP presidential hopefuls who attended Sunday's semiannual gathering of David and Charles Koch's donor network in California. Jim Cole/AP hide caption

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Rubio, seen here addressing the National Press Club in May, told NPR he'll decide on a presidential run in the next few months Alex Wong/Getty hide caption

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From left, Sen. Jeff Flake, R-Ariz., Sen. Chuck Schumer, D-NY, Sen. John McCain, R-Ariz., and Sen. Michael Bennett, D-Colo, arrive at a news conference after their tour of the Mexico border with the United States on Wednesday in Nogales, Ariz. The senators are part of the "Gang of Eight," a larger group of legislators collaborating on changes to immigration. Ross D. Franklin/AP hide caption

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Can our nation afford political waffling on issues of scientific consensus? Stan Honda/AFP/Getty Images hide caption

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Former President Clinton has campaigned on behalf of Rep. Kendrick Meek, the Democratic candidate for Senate in Florida.  John Raoux/AP hide caption

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