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Hillary Clinton picked up a major endorsement Thursday, one so critical to her campaign that she held a campaign rally to highlight it.

Secretary of the Department of Housing and Urban Development, Julian Castro, is the second person in President Barack Obama’s cabinet and first major Latino to endorse Clinton.

“Having watched and respected for years now Secretary Clinton, I know that she appeals to Americans of all backgrounds and colors, different perspectives and walks of life,” Castro said with Clinton standing next to him at an event in San Antonio, Texas.

Clinton, who is riding a wave of momentum after a well-regarded debate performance in Las Vegas Tuesday night, spoke to an enthusiastic crowd where she called Castro, who is the popular former mayor of San Antonio, a great leader who “gives a really good speech.”

His endorsement comes even as his colleague in the Obama administration, Vice President Joe Biden, has not ruled out a presidential run.

“I believe that he would also make a fantastic president,” Castro said of Biden on MSNBC’s “Andrea Mitchell Reports." “My support of Secretary Clinton is based on a long relationship that Joaquin and I have had with her as well as with former President Clinton.”

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Castro’s endorsement is also shrouded in speculation that he is a top contender for the vice president slot – should she win the nomination.

Clinton said she “thinks really highly” of Castro and is “really going to look hard at him for anything because that’s how good he is.”

Castro hasn’t commented publicly on the possibility but said on MSNBC that he “doubts that's going to happen.”

Clinton is relatively popular among Latinos. According to an NBC News/ Wall Street Journal poll, 53 percent of Latinos had a positive view of her and only 21 had a negative opinion. While her favorability ratings are higher among Latinos than with the general population, she still has some work to do. In 2008 she beat Obama in the primaries among Latinos, receiving 63 percent of their support.

Clinton is in Texas on a Latino swing. Earlier in the day she spoke at an event hosted by the Hispanic Chamber of Commerce where she said the Latino vote is “personal.”

“I know how important it is that my policies and my priorities reflect the real needs of the American people and not some ideology that’s not connected to the lives people are leading today,” she said.

Castro said Clinton’s strengths in the Hispanic community includes her long-standing career of leading “efforts to make the right kind of investments so that aspiring communities like the Latino community are able to get a good education, to afford college and then be able to enter the middle class by working hard.”

But some Latinos are unsure.

Cesar Vargas, co-director of The Dream Act Coalition, a Latino group that advocates for undocumented youth, said he is seeing a lot of momentum for Vermont Senator Bernie Sanders, noting that he’s released policies they support pertaining to immigrants.

“In terms of the overall debate, a lot of people in the community realized that Mrs. Clinton is playing a very safe political position compared to Sen. Sanders,” Vargas said.

Vargas pointed to Sanders’ support of providing Obamacare to undocumented immigrants and his opposition to using private prisons.

Rep. Joaquin Castro (Julian’s twin brother) has already endorsed Clinton and also attended today’s event, which was the campaign’s first “Latinos for Hillary” organizing event in the state. The campaign launched their Latino initiative earlier this month.