WASHINGTON — Houston congressman Al Green, a leading crusader for the impeachment of President Donald Trump, signed on as Texas co-chairman of Sen. Kamala Harris' presidential campaign Thursday.

The Harris campaign made no mention of Green's long support for impeachment in announcing the endorsement, focusing instead on his support for an immigration overhaul plan.

He is the second member of the Congressional Black Caucus to endorse the California Democrat.

While Green was one of the first Democrats to demand Trump's impeachment, Harris added her voice only three months ago — days after the release of the special counsel's report that cleared Trump of colluding with Russia in the 2016 election while also laying out evidence that he sought to obstruct justice in the subsequent probe of him and his aides.

"There is a lot of good evidence pointing to obstruction and obstruction of justice," Harris, a former California attorney general, said then. "I believe Congress should take the steps towards impeachment."

Impeachment has become the signature issue for Green, serving his eighth term.

It isn't clear if that's what drew him to her, though she alluded to it, saying he wants to ensure that "no one is above the law."

Her stump speech signals a tough stance with the line: "I am prepared to make the case for America, and to prosecute the case against Donald Trump."

When the Mueller investigation began, powerful voices indicated it was “too early” to impeach - wait for the Mueller report. Now that the report has been released, powerful voices are starting to indicate that it’s “too late” to impeach - wait for elections. 🤔#ImpeachmentNOW — Congressman Al Green (@RepAlGreen) April 20, 2019

She is one of two black candidates in the Democratic field, along with Sen. Cory Booker of New Jersey.

"Kamala Harris represents leadership and commitment to service that is desperately needed in the White House," Green said in a statement released by the campaign. "From creating a program to address trauma in children who witness domestic violence as San Francisco's District Attorney, to standing up to big banks and winning billions for California's homeowners as Attorney General, to being one of the foremost opponents in the Senate to the corruption of this Administration — Kamala Harris represents the future of our nation."

"I'm so excited to have Al's support in this race," Harris said. "He understands what our working and middle class families are facing each day and that we must restore trust in government to work in their best interest. This starts with a tax code that benefits the middle class instead of the wealthy to protecting and preserving all tenets of our democracy but assuring them that no one is above the law."

Green forced two unsuccessful House votes on impeachment and has vowed to press for more. In December 2017, the House defeated his impeachment resolution 364-58. In January 2018, the measure failed by a similar margin, 355-66. Green has cited Trump's ties to Russia, along with "invidious discrimination."

Two Texans are also running for the Democratic nomination, former El Paso congressman Beto O'Rourke, and Julian Castro, a former mayor of San Antonio and a housing secretary during the Obama administration, whose brother Joaquin Castro serves in the U.S. House.

Six of the 13 Texas Democrats in Congress have picked favorites among the two dozen Democrats vying for the White House.

Castro's brother serves as his campaign chairman. He also has endorsements from Rep. Colin Allred of Dallas, who worked under him at the Department of Housing and Urban Development, and from Rep. Vicente Gonzalez of McAllen.

O'Rourke is backed by his successor in the El Paso-based district, Rep. Veronica Escobar.

Former Vice President Joe Biden collected an endorsement from Rep. Filemon Vela of Brownsville five weeks ago.