Mayor Julian Castro testifies during his confirmation hearing before the Senate Banking, Housing and Urban Affairs Committee in the Dirksen Senate Office Building on Capitol Hill June 17, 2014 in Washington, DC.
CNN  — 

Julian Castro, a rising star in the Democratic party and a possible vice presidential candidate, will endorse Hillary Clinton in San Antonio next week, according to a Clinton spokesman.

Castro, the Secretary of Housing and Urban Development, is a native of San Antonio and served as the city’s mayor from 2009 to 2014.

The endorsement will come at a Latinos for Hillary grassroots event, Clinton’s first organizing event in Texas since announcing her candidacy earlier this year and part of her plan to focus on Latino voters for the month of October.

In addition to Castro, Clinton has also scored endorsements from Hispanic celebrities Marc Anthony and Salma Hayek, and Dolores Huerta, a longtime Hispanic labor organizer.

News of Castro’s endorsement was first reported by Buzzfeed.

Castro had dinner with former President Bill Clinton in August 2014, according to the Washington Post, a sign that many saw as the start of Castro’s rise to a potential vice presidential nomination.

Latinos are a critically important voting bloc for the Democratic Party, one that could possibly sway both the race for the party’s nomination and the general election. Clinton finds herself in a tighter than expected race with independent Vermont Sen. Bernie Sanders and her move to consolidate Latino support is a sign the candidate is trying to solidify her base.

Clinton has long performed well with Latino voters, too. During the 2008 Democratic primary, Hispanics picked Clinton over then-Sen. Barack Obama by a nearly two-to-one margin.

Republicans used the news as an opportunity to hit both the endorser and the recipient.

“Hillary Clinton and Julian Castro have one thing in common: no one has any idea what they’ve accomplished,” RNC Spokeswoman Ruth Guerra said in a statement. “This endorsement will do nothing to save Hillary’s flailing campaign in the midst of plummeting poll numbers and an FBI investigation into the mishandling of classified information on her secret email server.”

Castro is not the first member of his family to endorse Clinton, either. His brother, Rep. Joaquin Castro, has endorsed Clinton and is slated to campaign for her in Las Vegas, Nevada on Sunday, ahead of next week’s first Democratic debate.

Julian Castro has been a staunch defender of Clinton over the last few months. Asked by CNN in August about her growing email controversy, Castro said, “Folks need to understand that she did not handle classified information that was classified at that the time. It may have become classified later, but it was not classified that way at the time.”

He has also batted down any speculation about possibly becoming vice president.

“It’s flattering,” he said of the speculation at an event in Kansas City in July, “but there really is not anything to make of it. I’m focused on my work at HUD. This is not like something you run for. Nobody signs up for that. So I’m just gonna keep doing what I’m doing.”