*This story has been updated throughout.

U.S. Rep. Lloyd Doggett, D-Austin, will endorse former Secretary of State Hillary Clinton for the presidency.

Doggett confirmed his plans on Sunday; he is expected to appear with former President Bill Clinton at a Monday campaign event at the Guadalupe Cultural Arts Center Theater in San Antonio, located in the congressman's 35th District.

Doggett is a superdelegate. As a Democratic member of Congress, he is also an unpledged delegate to the Democratic national convention and can offer his support to the candidate he chooses — regardless of whom Texas voters support. In 2008, Doggett remained neutral during the divisive delegate fight between Clinton and then-U.S. Sen. Barack Obama. His endorsement means that every member of the Democratic federal delegation except U.S. Rep. Beto O'Rourke, D-El Paso, is publicly behind Clinton.

Clinton dominates the superdelegate race in Texas and the rest of the country.

So far, no Texas superdelegates back Sanders. His campaign argues that if a Clinton superdelegate's base constituency votes for Sanders, the campaign can make a plausible case to re-evaluate that stance before the Democratic National Committee's Convention in the summer.

An endorsement from a member of Congress can mean more than just a delegate toward the nomination. Each member has a granular understanding of his or her House district and knows better than anyone else how to turn out the Democratic vote in a primary. The Clinton camp is leaning heavily on these members, especially in the Dallas-Fort Worth and Houston areas.

Doggett's endorsement, to some extent, undercuts Sanders in what is expected to be the Vermont senator's Central Texas stronghold.

Doggett's district meanders from parts of Austin into San Antonio, thanks to district lines drawn by Republican state legislators. But throughout Austin and among University of Texas students, he is a dominant Democratic figure.