Once considered a potential candidate to unseat incumbent Greg Abbott, U.S. Rep. Joaquin Castro on Monday endorsed Lupe Valdez for governor.

Castro, D-San Antonio, said the former Dallas County sheriff is the best choice from a field of nine candidates to take on Abbott in November.

"Sheriff Lupe Valdez knows what it means to fight for hard-working Texans," Castro said in a prepared statement released by the Valdez campaign. "From migrant farmworker, to college student, Army veteran, federal agent and the sheriff of Dallas County, Lupe has spent her whole life working hard and defending Texans."

Valdez, 70, is running for the Democratic nomination for governor, and her central opposition in the March 6 primary is Houston investor Andrew White, the son of former Texas Gov. Mark White.

After a slow start, Valdez's campaign is starting to pick up momentum. She's hired a campaign manager and produced an online ad, just as early voting began.

With so many candidates in the race, the matter could be settled in a May runoff.

U.S. Representative Joaquin Castro speaks during a One Texas Resistance newsconference at the Texas state capitol in Austin. (Ashley Landis / Staff Photographer)

Castro, a former member of the Texas House who has served Congressional District 20 since 2013, was mentioned as a potential candidate for governor, as well as a candidate for Senate against incumbent Republican Ted Cruz.

But the lawmaker opted against running for Senate after the emergence of Rep. Beto O'Rourke, D-El Paso, as a serious contender to unseat Cruz.

In September, Texas Democratic Party Chairman Gilberto Hinojosa told The Dallas Morning News that Castro has not ruled out a run for governor. Ultimately, Castro opted to run for re-election, instead of a statewide campaign.

"I am proud to endorse Sheriff Lupe Valdez for Texas governor, because I know she will always stand up for us," Castro said.