U.S. Senator Marco Rubio (R-FL) autographs a magazine with his picture on the cover after he announced his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech at Freedom Tower in Miami, Florida April 13, 2015. REUTERS/Carlo Allegri TPX IMAGES OF THE DAY)

U.S. Senator Marco Rubio (R-FL) waves to the crowd after he announced his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech at Freedom Tower in Miami, Florida April 13, 2015. REUTERS/Carlo Allegri TPX IMAGES OF THE DAY

U.S. Senator Marco Rubio (R-FL) gives a TV interview after he announced his bid for the White House in 2016 at the Freedom Tower in Miami April 13, 2015. Rubio of Florida urged voters on Monday to make a "generational choice" for new leadership that is not mired in the policies of the past, as the Republican announced he will enter the 2016 race for the White House. REUTERS/Carlo Allegri

U.S. Senator Marco Rubio (R-FL) announces his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech in Miami, Florida April 13, 2015. Rubio, 43, announced his candidacy at Miami's Freedom Tower, where thousands of Cuban exiles fleeing the communist-run island in the 1960s were first registered by U.S. authorities. REUTERS/Carlo Allegri

U.S. Senator Marco Rubio (R-FL) announces his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech in Miami, Florida April 13, 2015. Rubio, 43, announced his candidacy at Miami's Freedom Tower, where thousands of Cuban exiles fleeing the communist-run island in the 1960s were first registered by U.S. authorities. REUTERS/Carlo Allegri

U.S. Senator Marco Rubio (R-FL) announces his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech in Miami, Florida April 13, 2015. Rubio, 43, announced his candidacy at Miami's Freedom Tower, where thousands of Cuban exiles fleeing the communist-run island in the 1960s were first registered by U.S. authorities. REUTERS/Carlo Allegri

U.S. Senator Marco Rubio (R-FL) announces his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech in Miami, Florida April 13, 2015. Rubio, 43, announced his candidacy at Miami's Freedom Tower, where thousands of Cuban exiles fleeing the communist-run island in the 1960s were first registered by U.S. authorities. REUTERS/Carlo Allegri

Senator Marco Rubio (R-FL) speaks at the Council on Foreign Relations in New York May 31, 2012. REUTERS/Eric Thayer (UNITED STATES - Tags: POLITICS HEADSHOT) 

U.S. Senator Marco Rubio (R-FL) is interviewed at the Reuters Health Summit 2014 in Washington April 1, 2014. REUTERS/Gary Cameron (UNITED STATES - Tags: HEALTH BUSINESS POLITICS HEADSHOT) 

A crowd of supporters waits for U.S. Senator Marco Rubio's formal declaration of his bid for the
Republican nomination in the 2016 U.S. presidential election race, inside the Freedom Tower in Miami, Florida April 13, 2015. REUTERS/Joe Skipper

Campaign buttons for U.S. Senator Marco Rubio (R-FL) are picture prior to Rubio's formal declaration of his bid for the
Republican nomination in the 2016 U.S. presidential election race, at the Freedom Tower in Miami, Florida April 13, 2015. REUTERS/Carlo Allegri

A supporter waits to have a book autographed after U.S. Senator Marco Rubio (R-FL) announced his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech at the Freedom Tower in Miami, Florida April 13, 2015. REUTERS/Carlo Allegri

U.S. Senator Marco Rubio (R-FL) (L) greets supporters after he announced his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech at Freedom Tower in Miami, Florida April 13, 2015. REUTERS/Carlo Allegri

U.S. Senator Marco Rubio (R-FL) waves on stage with his family after he announced his bid for the White House in 2016 at the Freedom Tower in Miami April 13, 2015. Rubio of Florida urged voters on Monday to make a "generational choice" for new leadership that is not mired in the policies of the past, as the Republican announced he will enter the 2016 race for the White House. REUTERS/Carlo Allegri

U.S. Senator Marco Rubio (R-FL) high fives his son Dominick on stage after he announced his bid for the White House in 2016 at the Freedom Tower in Miami April 13, 2015. Rubio of Florida urged voters on Monday to make a "generational choice" for new leadership that is not mired in the policies of the past, as the Republican announced he will enter the 2016 race for the White House. REUTERS/Carlo Allegri

U.S. Senator Marco Rubio (R-FL) announces his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech in Miami, Florida April 13, 2015. Rubio, 43, announced his candidacy at Miami's Freedom Tower, where thousands of Cuban exiles fleeing the communist-run island in the 1960s were first registered by U.S. authorities. REUTERS/Carlo Allegri

U.S. Senator Marco Rubio (R-FL) announces his bid for the Republican nomination in the 2016 U.S. presidential election race during a speech in Miami, Florida April 13, 2015. Rubio, 43, announced his candidacy at Miami's Freedom Tower, where thousands of Cuban exiles fleeing the communist-run island in the 1960s were first registered by U.S. authorities. REUTERS/Carlo Allegri

Florida Senator Marco Rubio speaks at the Conservative Political Action Conference (CPAC) at National Harbor in Maryland February 27, 2015. REUTERS/Kevin Lamarque (UNITED STATES - Tags: POLITICS) 

Republican U.S. Senator and possible presidential candidate for 2016 Marco Rubio (R-FL) (R) speaks during an appearance in Hollis, New Hampshire February 23, 2015. REUTERS/Brian Snyder (UNITED STATES - Tags: POLITICS) 

U.S. Senator Marco Rubio (R-FL) (L), the chair of the Senate Foreign Relations' Western Hemisphere Subcommittee, talks with Senator Barbara Boxer (D-CA) during a hearing on President Barack Obama's changes to Cuba policy in Washington February 3, 2015. REUTERS/Gary Cameron (UNITED STATES - Tags: POLITICS) 

U.S. Senator Marco Rubio (L) shakes hands with Japan's Prime Minister Shinzo Abe (R) before their talks at Abe's office in Tokyo January 21, 2014. Rubio, a member of U.S. Senate Foreign Relations Committee is in Japan to exchange views with Japanese officials. REUTERS/Yoshikazu Tsuno/Pool (JAPAN - Tags: POLITICS) 

Republican presidential nominee Mitt Romney shakes hands with U.S. Senator Marco Rubio (R-FL) (L) at a campaign rally in Coral Gables, Florida October 31, 2012. REUTERS/Brian Snyder (UNITED STATES - Tags: POLITICS ELECTIONS USA PRESIDENTIAL ELECTION) 

U.S. Senator Marco Rubio (R-FL) addresses delegates as he introduces Republican presidential nominee Mitt Romney during the final session of the Republican National Convention in Tampa, Florida, August 30, 2012 REUTERS/Adrees Latif (UNITED STATES - Tags: POLITICS ELECTIONS)

U.S. Senator Marco Rubio (R-FL) addresses the American Conservative Union's annual Conservative Political Action Conference (CPAC) in Washington, February 9, 2012. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS) 

U.S. Senator Marco Rubio (R-FL) (L) stands next to his wife Jeanette Rubio as he takes part in a ceremonial re-enactment of his swearing-in by Vice President Joe Biden in the Old Senate Chamber at the U.S. Capitol in Washington January 5, 2011. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS) 

U.S. Republican Senate candidate Marco Rubio (L) celebrates with his mother Oria (R) during his victory rally in Coral Gables, Florida, November 2, 2010. Republicans rolled up key election wins on Tuesday, putting them on the threshold of taking control of the House of Representatives. REUTERS/Hans Deryk (UNITED STATES - Tags: ELECTIONS POLITICS) 

U.S. Republican Senate candidate Marco Rubio (L) stands with his children Daniella (2nd L), Amanda (2nd R) and Dominic (R) during his victory speech at a rally in Coral Gables, Florida November 2, 2010. REUTERS/Hans Deryk (UNITED STATES - Tags: ELECTIONS POLITICS) 

U.S. Senate candidate Marco Rubio (R-FL) smiles during a public event to sign the election documents to officially qualify as a Republican party candidate for the U.S. Senate during a event in West Miami, Florida April 27, 2010. Former U.S. Vice President Dick Cheney endorsed Republican Marco Rubio in Florida's U.S. Senate race and coupled his announcement with sharp criticism of Rubio's competitor, Governor Charlie Crist. As conservative favorite Rubio picks up momentum before the August 31 primary, support for Crist has been dropping in the Republican Party to the extent that the Florida governor has been contemplating whether to run as an independent. REUTERS/Carlos Barria (UNITED STATES - Tags: POLITICS)