Rep. Joaquin Castro Joaquin CastroHispanic Caucus members embark on 'virtual bus tour' with Biden campaign Hispanic caucus report takes stock of accomplishments with eye toward 2021 Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-Texas) is reportedly closing in on a bid to challenge Sen. John Cornyn John CornynThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Senate GOP faces pivotal moment on pick for Supreme Court MORE (R-Texas) for his Senate seat in 2020.

Texas Monthly reported Friday that the San Antonio Democrat is “all but certain” to enter next year’s Senate contest, citing a source familiar with his thinking.

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Matthew Jones, a campaign adviser to Castro, said in an email that the congressman “will have an announcement in the near future.”

If Castro enters the race, he would become the first high-profile candidate to mount a campaign against Cornyn, a three-term Republican and former Senate majority whip with a well-financed political operation.

Cornyn's campaign closed out 2018 with a nearly $5.8 million campaign bank account.

Meanwhile, Castro’s House campaign finished the year with $128,000 on hand, according to his most recent filing with the Federal Election Commission (FEC).

Cornyn's campaign office said it was looking forward to a potential challenge from Castro.

"Whether it’s Hurricane Harvey relief or the Green New Deal, time and again Congressman Castro has stood with Nancy Pelosi Nancy PelosiPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare House lawmakers reach deal to avert shutdown Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill MORE at the expense of Texans,” said John Jackson, Cornyn's campaign manager, 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.”

Castro has been floated for months as a potential challenger to Cornyn. But speculation of a Senate campaign had been overshadowed recently by rumors that former Rep. Beto O’Rourke (D-Texas), whose 2018 challenge to Sen. Ted Cruz Rafael (Ted) Edward CruzTrump argues full Supreme Court needed to settle potential election disputes Press: Notorious RBG vs Notorious GOP The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Texas) drew intense national attention, could look to take on Cornyn instead.

O’Rourke ended months of speculation about his political future on Thursday, announcing that he would run for president in 2020 and leaving the Democratic field in the Texas Senate race wide open.

Among O'Rourke's fellow hopefuls for the White House nomination is Castro's twin brother, former Housing and Urban Development Secretary Julián Castro.

News of a possible Senate announcement comes a day after the Senate passed a resolution overturning President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s national emergency declaration seeking funding to construct his long-promised wall along the U.S.-Mexico border.

That vote was seen as a victory for Castro, who initiated the resolution in the House. It passed in the lower chamber last month.

Trump vetoed the measure on Friday, the first veto of his presidency.

-- Updated at 6:32 p.m.