Republican U.S. presidential candidate Donald Trump speaks at his campaign rally in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford

Republican U.S. presidential candidate Donald Trump speaks at his campaign rally in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford

  Supporters recite the Pledge of Allegiance at a rally for Republican U.S. presidential candidate Donald Trump in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford

Supporters recite the Pledge of Allegiance at a rally for Republican U.S. presidential candidate Donald Trump in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford

  Protestors hold up a sign towards the crowd at a rally for U.S. Republican presidential candidate Donald Trump at Oral Roberts University in Tulsa, Oklahoma, January 20, 2016. REUTERS/Nick Oxford

Protestors hold up a sign towards the crowd at a rally for U.S. Republican presidential candidate Donald Trump at Oral Roberts University in Tulsa, Oklahoma, January 20, 2016. REUTERS/Nick Oxford

  Republican U.S. presidential candidate Marco Rubio acknowledges supporters while speaking at a campaign rally in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford

Republican U.S. presidential candidate Marco Rubio acknowledges supporters while speaking at a campaign rally in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford

 Presidential candidate Marco Rubio gives a speech unveiling his energy plan at the Oklahoma Independent Petroleum Association in Oklahoma City.

Presidential candidate Marco Rubio gives a speech unveiling his energy plan at the Oklahoma Independent Petroleum Association in Oklahoma City.

  A sign directs caucus goers to the home of Gary and Mary Weaver in Rippey, Iowa February 1, 2016. REUTERS/Nick Oxford

A sign directs caucus goers to the home of Gary and Mary Weaver in Rippey, Iowa February 1, 2016. REUTERS/Nick Oxford

USA-ELECTION
USA-ELECTION

Bert Rushmore leaves a rural polling site after voting on Super Tuesday in Stillwater, Oklahoma March 1, 2016. REUTERS/Nick Oxford

 A discarded "I Voted" sticker is pictured outside a rural polling site on Super Tuesday in Stillwater, Oklahoma March 1, 2016. REUTERS/Nick Oxford

A discarded "I Voted" sticker is pictured outside a rural polling site on Super Tuesday in Stillwater, Oklahoma March 1, 2016. REUTERS/Nick Oxford

 Democrat presidential candidate Bernie Sanders speaks to supporters at a rally in Tulsa Oklahoma.

Democrat presidential candidate Bernie Sanders speaks to supporters at a rally in Tulsa Oklahoma.

 Democrat presidential candidate Bernie Sanders speaks to supporters at a rally in Des Moines Iowa on the eve of the Iowa Caucuses.

Democrat presidential candidate Bernie Sanders speaks to supporters at a rally in Des Moines Iowa on the eve of the Iowa Caucuses.

 Former U.S. President Bill Clintion speaks to a crowd of supporters at a rally for his wife and Democrat presidential candidate Hillary Clinton in Oklahoma City.

Former U.S. President Bill Clintion speaks to a crowd of supporters at a rally for his wife and Democrat presidential candidate Hillary Clinton in Oklahoma City.

  Ethan Lang, 18, shows his support for U.S. Republican presidential candidate Donald Trump at a campaign rally in Oklahoma City, Oklahoma February 26, 2016.  Reuters/Nick Oxford

Ethan Lang, 18, shows his support for U.S. Republican presidential candidate Donald Trump at a campaign rally in Oklahoma City, Oklahoma February 26, 2016. Reuters/Nick Oxford

 A campaign volunteer takes down the American flag after a rally for Republican presidential candidate Marco Rubio in Oklahoma City on the eve of Super Tuesday.

A campaign volunteer takes down the American flag after a rally for Republican presidential candidate Marco Rubio in Oklahoma City on the eve of Super Tuesday.

 Republican presidential candidate Ted Cruz greets supporters at a rally in Oklahoma City. Reuters/Nick Oxford

Republican presidential candidate Ted Cruz greets supporters at a rally in Oklahoma City. Reuters/Nick Oxford

 Maggie Wright waves a sign in support of republican presidential candidate Senator Ted Cruz at a rally in Oklahoma City, Oklahoma August 13, 2015.

Maggie Wright waves a sign in support of republican presidential candidate Senator Ted Cruz at a rally in Oklahoma City, Oklahoma August 13, 2015.

  Supporters listen to Republican U.S. presidential candidate Ted Cruz speak at a campaign rally in Oklahoma City, Oklahoma February 28, 2016. REUTERS/Nick Oxford

Supporters listen to Republican U.S. presidential candidate Ted Cruz speak at a campaign rally in Oklahoma City, Oklahoma February 28, 2016. REUTERS/Nick Oxford

 A member of Republican presidential candidate Ted Cruz's security detail keeps watch during a rally in Oklahoma City.

A member of Republican presidential candidate Ted Cruz's security detail keeps watch during a rally in Oklahoma City.

 Democrat presidential candidate Martin O'Malley embraces a supporter at a door knocking event the day before the Iowa Caucuses in Des Moines Iowa.

Democrat presidential candidate Martin O'Malley embraces a supporter at a door knocking event the day before the Iowa Caucuses in Des Moines Iowa.

 Supporters of Republican presidential candidate Ben Carson pose in front of his campaign bus at an event in Edmond Oklahoma.

Supporters of Republican presidential candidate Ben Carson pose in front of his campaign bus at an event in Edmond Oklahoma.

 Republican U.S. presidential candidate Donald Trump speaks at his campaign rally in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford
  Supporters recite the Pledge of Allegiance at a rally for Republican U.S. presidential candidate Donald Trump in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford
  Protestors hold up a sign towards the crowd at a rally for U.S. Republican presidential candidate Donald Trump at Oral Roberts University in Tulsa, Oklahoma, January 20, 2016. REUTERS/Nick Oxford
  Republican U.S. presidential candidate Marco Rubio acknowledges supporters while speaking at a campaign rally in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford
 Presidential candidate Marco Rubio gives a speech unveiling his energy plan at the Oklahoma Independent Petroleum Association in Oklahoma City.
  A sign directs caucus goers to the home of Gary and Mary Weaver in Rippey, Iowa February 1, 2016. REUTERS/Nick Oxford
USA-ELECTION
 A discarded "I Voted" sticker is pictured outside a rural polling site on Super Tuesday in Stillwater, Oklahoma March 1, 2016. REUTERS/Nick Oxford
 Democrat presidential candidate Bernie Sanders speaks to supporters at a rally in Tulsa Oklahoma.
 Democrat presidential candidate Bernie Sanders speaks to supporters at a rally in Des Moines Iowa on the eve of the Iowa Caucuses.
 Former U.S. President Bill Clintion speaks to a crowd of supporters at a rally for his wife and Democrat presidential candidate Hillary Clinton in Oklahoma City.
  Ethan Lang, 18, shows his support for U.S. Republican presidential candidate Donald Trump at a campaign rally in Oklahoma City, Oklahoma February 26, 2016.  Reuters/Nick Oxford
 A campaign volunteer takes down the American flag after a rally for Republican presidential candidate Marco Rubio in Oklahoma City on the eve of Super Tuesday.
 Republican presidential candidate Ted Cruz greets supporters at a rally in Oklahoma City. Reuters/Nick Oxford
 Maggie Wright waves a sign in support of republican presidential candidate Senator Ted Cruz at a rally in Oklahoma City, Oklahoma August 13, 2015.
  Supporters listen to Republican U.S. presidential candidate Ted Cruz speak at a campaign rally in Oklahoma City, Oklahoma February 28, 2016. REUTERS/Nick Oxford
 A member of Republican presidential candidate Ted Cruz's security detail keeps watch during a rally in Oklahoma City.
 Democrat presidential candidate Martin O'Malley embraces a supporter at a door knocking event the day before the Iowa Caucuses in Des Moines Iowa.
 Supporters of Republican presidential candidate Ben Carson pose in front of his campaign bus at an event in Edmond Oklahoma.

Republican U.S. presidential candidate Donald Trump speaks at his campaign rally in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford

Supporters recite the Pledge of Allegiance at a rally for Republican U.S. presidential candidate Donald Trump in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford

Protestors hold up a sign towards the crowd at a rally for U.S. Republican presidential candidate Donald Trump at Oral Roberts University in Tulsa, Oklahoma, January 20, 2016. REUTERS/Nick Oxford

Republican U.S. presidential candidate Marco Rubio acknowledges supporters while speaking at a campaign rally in Oklahoma City, Oklahoma February 26, 2016. REUTERS/Nick Oxford

Presidential candidate Marco Rubio gives a speech unveiling his energy plan at the Oklahoma Independent Petroleum Association in Oklahoma City.

A sign directs caucus goers to the home of Gary and Mary Weaver in Rippey, Iowa February 1, 2016. REUTERS/Nick Oxford

USA-ELECTION

Bert Rushmore leaves a rural polling site after voting on Super Tuesday in Stillwater, Oklahoma March 1, 2016. REUTERS/Nick Oxford

A discarded "I Voted" sticker is pictured outside a rural polling site on Super Tuesday in Stillwater, Oklahoma March 1, 2016. REUTERS/Nick Oxford

Democrat presidential candidate Bernie Sanders speaks to supporters at a rally in Tulsa Oklahoma.

Democrat presidential candidate Bernie Sanders speaks to supporters at a rally in Des Moines Iowa on the eve of the Iowa Caucuses.

Former U.S. President Bill Clintion speaks to a crowd of supporters at a rally for his wife and Democrat presidential candidate Hillary Clinton in Oklahoma City.

Ethan Lang, 18, shows his support for U.S. Republican presidential candidate Donald Trump at a campaign rally in Oklahoma City, Oklahoma February 26, 2016. Reuters/Nick Oxford

A campaign volunteer takes down the American flag after a rally for Republican presidential candidate Marco Rubio in Oklahoma City on the eve of Super Tuesday.

Republican presidential candidate Ted Cruz greets supporters at a rally in Oklahoma City. Reuters/Nick Oxford

Maggie Wright waves a sign in support of republican presidential candidate Senator Ted Cruz at a rally in Oklahoma City, Oklahoma August 13, 2015.

Supporters listen to Republican U.S. presidential candidate Ted Cruz speak at a campaign rally in Oklahoma City, Oklahoma February 28, 2016. REUTERS/Nick Oxford

A member of Republican presidential candidate Ted Cruz's security detail keeps watch during a rally in Oklahoma City.

Democrat presidential candidate Martin O'Malley embraces a supporter at a door knocking event the day before the Iowa Caucuses in Des Moines Iowa.

Supporters of Republican presidential candidate Ben Carson pose in front of his campaign bus at an event in Edmond Oklahoma.

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