Hillary Clinton addresses the Pennsylvania AFL-CIO three weeks before the Democratic primary contest against Barack Obama. Seated at right is Pennsylvania AFSCME President Gerald W. McEntee. Charles Dharapak/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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Reporters and campaign staffers rush to their cars to get to Hillary Clinton's first Iowa campaign stop. Tamara Keith/NPR hide caption

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It's been 35 years since Ronald Reagan announced his run for president. He did so just two months before the Iowa caucuses, unlike the protracted campaigning that takes place today. WJZ/AP hide caption

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Hillary Clinton begins to speak as her husband, former President Bill Clinton, moves to take a seat after introducing her at the Clinton Global Initiative on Sept. 22, 2014, in New York. Mark Lennihan/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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Sen. Marco Rubio, R-Fla., addresses an event held by the American Enterprise Institute for Public Policy Research in January 2014 in Washington. Alex Wong/Getty Images hide caption

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Hillary Clinton is running with all the advantages and challenges of someone who has been in the public eye for more than two decades. Andrew Burton/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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Hillary Clinton has described herself as the most famous person you don't really know. And as she launches into her second presidential campaign, she'll be reintroducing herself to voters who largely think they have her figured out. Mel Evans/AP hide caption

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Hillary Clinton in June 1969 at the Rodham family home. She was featured in a Life magazine story called "The Class of '69." Lee Balterman/The LIFE Premium Collection/Getty Images hide caption

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In an interview with NPR's Morning Edition host Steve Inskeep, President Obama said Wisconsin Gov. Scott Walker is advocating a "foolish approach" to Iran and that he should "bone up on foreign policy." Mito Habe-Evans/NPR hide caption

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"I'm doing something. You know, people can complain about the government all they want, but you see very few people actually attempting to do anything," Ryan Shepard said of running for president. Sean McGinnis Scanlon/Courtesy of Shepard 2016 Campaign hide caption

itoggle caption Sean McGinnis Scanlon/Courtesy of Shepard 2016 Campaign