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Image 2 of 33 CHARLOTTE, NC - SEPTEMBER 02: San Antonio Mayor Julian Castro stands on stage during preparations for the Democratic National Convention at Time Warner Cable Arena on September 2, 2012 in Charlotte, North Carolina. The DNC that will start on September 4 and run through September 7, will nominate U.S. President Barack Obama as the Democratic presidential candidate. less CHARLOTTE, NC - SEPTEMBER 02: San Antonio Mayor Julian Castro stands on stage during preparations for the Democratic National Convention at Time Warner Cable Arena on September 2, 2012 in Charlotte, North ... more Photo: Streeter Lecka / Getty Images

Image 3 of 33 San Antonio, Texas Mayor Julian Castro talks to reporters at the Democratic National Convention in Charlotte, N.C., Tuesday, Sept. 4, 2012. The mayor will give the keynote speech at the DNC. San Antonio, Texas Mayor Julian Castro talks to reporters at the Democratic National Convention in Charlotte, N.C., Tuesday, Sept. 4, 2012. The mayor will give the keynote speech at the DNC. Photo: Jae C. Hong / Associated Press

Image 4 of 33 San Antonio Mayor Julian Castro, left, who will be the convention keynote speaker, and his twin brother, state Rep. Joaquin Castro, who is running for U.S. Congress, are interviewed at the Democratic National Convention in Charlotte, N.C., Monday, Sept. 3, 2012. less San Antonio Mayor Julian Castro, left, who will be the convention keynote speaker, and his twin brother, state Rep. Joaquin Castro, who is running for U.S. Congress, are interviewed at the Democratic National ... more Photo: Charles Dharapak / Associated Press

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Image 6 of 33 San Antonio Mayor Julian Castro, left, who will be the convention keynote speaker, and his twin brother, state Rep. Joaquin Castro, who is running for U.S. Congress, are interviewed at the Democratic National Convention in Charlotte, N.C., Monday, Sept. 3, 2012. less San Antonio Mayor Julian Castro, left, who will be the convention keynote speaker, and his twin brother, state Rep. Joaquin Castro, who is running for U.S. Congress, are interviewed at the Democratic National ... more Photo: Charles Dharapak / Associated Press

Image 7 of 33 Mayoral candidate Julian Castro smiles after announcing his runoff election against Phil Hardberger at his campaign headquarters in the evening on Saturday, May 7, 2005. Castro was leading the early voting with 39 percent of the votes. less Mayoral candidate Julian Castro smiles after announcing his runoff election against Phil Hardberger at his campaign headquarters in the evening on Saturday, May 7, 2005. Castro was leading the early voting ... more Photo: JERRY LARA / SAN ANTONIO EXPRESS-NEWS

Image 8 of 33 San Antonio Mayor Julian Castro and his brother Joaquin Castro, right, wave to the Democratic National Convention in Charlotte, N.C., on Tuesday, Sept. 4, 2012. (AP Photo/J. Scott Applewhite) San Antonio Mayor Julian Castro and his brother Joaquin Castro, right, wave to the Democratic National Convention in Charlotte, N.C., on Tuesday, Sept. 4, 2012. (AP Photo/J. Scott Applewhite) Photo: J. Scott Applewhite / Associated Press

Image 9 of 33 San Antonio Mayor Julian Castro, left, and his brother Joaquin Castro, wave at delegates at the Democratic National Convention in Charlotte, N.C., on Tuesday, Sept. 4, 2012. (AP Photo/Lynne Sladky) San Antonio Mayor Julian Castro, left, and his brother Joaquin Castro, wave at delegates at the Democratic National Convention in Charlotte, N.C., on Tuesday, Sept. 4, 2012. (AP Photo/Lynne Sladky) Photo: Lynne Sladky / Associated Press

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Image 11 of 33 San Antonio Mayor Julian Castro, left, hugs his brother Joaquin Castro, at the Democratic National Convention in Charlotte, N.C., on Tuesday, Sept. 4, 2012. (AP Photo/Charlie Neibergall) San Antonio Mayor Julian Castro, left, hugs his brother Joaquin Castro, at the Democratic National Convention in Charlotte, N.C., on Tuesday, Sept. 4, 2012. (AP Photo/Charlie Neibergall) Photo: Charlie Neibergall / Associated Press

Image 12 of 33 San Antonio Mayor Julian Castro addresses the Democratic National Convention in Charlotte, N.C., on Tuesday, Sept. 4, 2012. (AP Photo/J. Scott Applewhite) San Antonio Mayor Julian Castro addresses the Democratic National Convention in Charlotte, N.C., on Tuesday, Sept. 4, 2012. (AP Photo/J. Scott Applewhite) Photo: J. Scott Applewhite / Associated Press

Image 13 of 33 Mexican President Enrique Pena Nieto met with San Antonio Mayor Julian Castro Thursday, Feb. 21, 2013, at Los Pinos, the Mexican president's residence. Mexican President Enrique Pena Nieto met with San Antonio Mayor Julian Castro Thursday, Feb. 21, 2013, at Los Pinos, the Mexican president's residence. Photo: Official photo

Image 14 of 33 Councilman Julian Castro, center, candidate for San Antonio mayor, addresses the crowd Tuesday, June 7, 2005, in San Antonio at Castro's campaign headquarters. At left is his girlfriend, Erica Lira, and at right is his twin brother, Joaquin. Retired state judge Phil Hardberger held a narrow lead over Castro late Tuesday in the mayoral runoff. less Councilman Julian Castro, center, candidate for San Antonio mayor, addresses the crowd Tuesday, June 7, 2005, in San Antonio at Castro's campaign headquarters. At left is his girlfriend, Erica Lira, and at ... more Photo: BAHRAM MARK SOBHANI / AP

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Image 16 of 33 San Antonio Mayor Julian Castro, left, and his brother, state Rep. Joaquin Castro give an interview during preparations for the Democratic National Convention at Time Warner Cable Arena on Sept. 3, 2012 in Charlotte, N.C. less San Antonio Mayor Julian Castro, left, and his brother, state Rep. Joaquin Castro give an interview during preparations for the Democratic National Convention at Time Warner Cable Arena on Sept. 3, 2012 in ... more Photo: Streeter Lecka / Getty Images

Image 17 of 33 In the eye of the camera: Julian Castro. In the eye of the camera: Julian Castro. Photo: Rick Dunham / Hearst Newspapers

Image 18 of 33 Mayor Castro answers reporters' questions at the Democratic National Convention. Mayor Castro answers reporters' questions at the Democratic National Convention. Photo: Rick Dunham / Hearst Newspapers

Image 19 of 33 Julian Castro at a Democratic National Committee session. Julian Castro at a Democratic National Committee session. Photo: Rick Dunham / Hearst Newspapers

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Image 21 of 33 Julian Castro addresses the National Association of Latino Elected and Appointed Officials (NALEO). Julian Castro addresses the National Association of Latino Elected and Appointed Officials (NALEO). Photo: AP

Image 22 of 33 Julian Castro (in the blue tie) shakes hands with President Obama on July 17. The San Antonio mayor is flanked by his twin brother Joaquin. Julian Castro (in the blue tie) shakes hands with President Obama on July 17. The San Antonio mayor is flanked by his twin brother Joaquin. Photo: Bahram Mark Sobhani / AP

Image 23 of 33 President Obama walks from Air Force One with Texas State Rep. Joaquin Castro, his twin brother the mayor and Rep. Charlie Gonzalez, upon his arrival for a fundraising visit to San Antonio on July 17. President Obama walks from Air Force One with Texas State Rep. Joaquin Castro, his twin brother the mayor and Rep. Charlie Gonzalez, upon his arrival for a fundraising visit to San Antonio on July 17. Photo: Bahram Mark Sobhani / AP

Image 24 of 33 San Antonio Mayor Julian Castro gives the keynote address at the Texas Democratic Convention in Houston on June 8. San Antonio Mayor Julian Castro gives the keynote address at the Texas Democratic Convention in Houston on June 8. Photo: Pat Sullivan / AP

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Image 26 of 33 Julian Castro easily won a second term as mayor. Julian Castro easily won a second term as mayor. Photo: San Antonio Express-News

Image 27 of 33 Julian Castro smiles as an audience applauds him. Julian Castro smiles as an audience applauds him. Photo: San Antonio Express-News

Image 28 of 33 Democrats view Julian Castro as a future star of the national party. Democrats view Julian Castro as a future star of the national party. Photo: San Antonio Express-News

Image 29 of 33 Mayor Castro is considered one of his party's best political orators. Mayor Castro is considered one of his party's best political orators. Photo: San Antonio Express-News

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Image 31 of 33 Joaquin Castro left, introduces his twin brother and San Antonio Mayor Julian Castro during the the 2012 Texas Democratic Party State Convention at the George R. Brown Convention Center Friday, June 8, 2012, in Houston. less Joaquin Castro left, introduces his twin brother and San Antonio Mayor Julian Castro during the the 2012 Texas Democratic Party State Convention at the George R. Brown Convention Center Friday, June 8, 2012, ... more Photo: James Nielsen / Chronicle

Image 32 of 33 Democratic Congressional Candidate Joaquin Castro speaks prior to introducing his twin brother and San Antonio Mayor Julian Castro during the the 2012 Texas Democratic Party State Convention at the George R. Brown Convention Center Friday, June 8, 2012, in Houston. less Democratic Congressional Candidate Joaquin Castro speaks prior to introducing his twin brother and San Antonio Mayor Julian Castro during the the 2012 Texas Democratic Party State Convention at the George R. ... more Photo: James Nielsen / Chronicle