Joaquin Castro appears likely to challenge U.S. Sen. John Cornyn

U.S. Rep. Joaquin Castro appears likely to forgo re-election next year and instead challenge U.S. Sen. John Cornyn, R-Texas.

According to a source close to Castro, while the congressman has not finalized his decision, his mind "is virtually made up” to take on Cornyn.

Matthew Jones, Castro's campaign adviser, told The Texas Tribune that an announcement on Castro's decision will come "in the near future."

The news comes as President Donald Trump vetoed a resolution authored by Castro to block the president's emergency declaration to build a wall along the U.S-Mexico border. Both Cornyn and Texas' other senator, Ted Cruz, voted against the resolution.

Earlier Friday afternoon, Texas Monthly reported that Castro was "all but certain" to take on Cornyn, citing a source familiar to Castro's thinking.

Castro revealed two weeks ago that he was strongly eyeing a U.S. Senate bid — days after El Paso Democrat Beto O’Rourke, who launched a bid for president this week, announced that he would not be competing for Cornyn's seat. Castro's twin brother, former Housing and Urban Development Secretary Julián Castro, is also seeking the 2020 Democratic presidential nomination.

John Jackson, Cornyn's campaign manager, said Cornyn is ready to contrast his record to Castro's.

“Whether it’s Hurricane Harvey relief or the Green New Deal, time and again Congressman Castro has stood with Nancy Pelosi at the expense of Texans,” Jackson said in a statement. “John Cornyn looks forward to contrasting the Democrat-Socialist agenda with the policies that have made Texas the best state to live, work, and raise a family.”

Patrick Svitek and Abby Livingston contributed to this report.