Joaquin Castro, the Democratic congressman from San Antonio, “is all but certain” to enter next year’s race for U.S. Senate and take on incumbent Republican John Cornyn, a source familiar with Castro’s thinking said Thursday.

The move would profoundly change the dynamics of the 2020 campaign and put Texas squarely on center stage, with two Texans already in the Democratic primary race for president and Joaquin taking on a longtime Republican senator who many see as vulnerable, especially during a presidential election year.

“We’ll be making an announcement in the very near future,” said Matthew Jones, Castro’s campaign adviser.

“This instantly makes the race very competitive,” said Bill Miller, an Austin lobbyist and longtime political observer, of Joaquin’s potential entry into the race. Running in tandem with his brother, Julián, who announced his candidacy for president on January 12 in San Antonio, would only benefit both candidates, Miller said, and “doubles up on all the positives.” When asked if Cornyn was vulnerable, Miller said, “Every Republican senator up for election next year is vulnerable.”

“This is quite an important development,” said Larry Sabato, a political scientist at the University of Virginia. “Beto proved Texas can be competitive, and this means that Cornyn is really going to have to work hard to raise money and work hard to earn votes—and Republicans in Texas are not used to doing that.”

Since most of his tenure in the U.S. House of Representatives, Castro has been in the minority party and has not compiled much of a legislative record. But his focus as Democrats took control of the House this year has been on foreign policy issues, and he has taken an aggressive stance on President Trump’s immigration policies. That allows him to contrast himself with Cornyn, who has defended Trump and his border wall, although Cornyn himself has espoused a much more moderate position on a wall, often suggesting the utilization of either technology or a physical wall depending on where each is more effective.

“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,” Cornyn campaign manager John Jackson said in response to a request for comment. “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.”

Joaquin Castro’s candidacy also raises the prospects of more money flowing into Texas, Sabato said. “Even if a Texan isn’t on the national ticket, Democrats at the presidential level have every incentive to put money into Texas even if it’s to give Trump a scare,” Sabato said. He also believes that the Democratic Senatorial Campaign Committee, which helps fund races in states where Democrats have a chance of winning, will also begin investing in Texas for the first time in more than two decades.

The source said a timeline has not been established for Joaquin to formalize any announcement, but one Democrat who did not want to be identified because he was not authorized to speak on behalf of the congressman said that Joaquin has been reaching out and telling several key Democratic leaders in Texas that he has been leaning toward running. Castro’s decision may have further solidified on Thursday after O’Rourke announced he was running for president. There had been speculation that O’Rourke may have taken on Cornyn following his 2.5 percentage point defeat to Republican Ted Cruz last year.

Mark McKinnon, a former adviser to President George W. Bush and currently a producer and writer for Showtime’s The Circus: Inside the Greatest Political Show on Earth, said shifting demographics will play a major role in next year’s election. “With demographic trends shifting toward Democrats in Texas, there’s only two ways for Republicans to run statewide anymore: unopposed or scared,” McKinnon said. “If Joaquin Castro gets into Cornyn’s rear-view mirror, the senior senator better step on the gas.”

Castro enjoyed a significant and historic political victory on Thursday. The Senate voted 59-41 on a resolution that Castro initiated and got passed in the House to overturn Trump’s emergency declaration, which would allow the president to circumvent Congress and get additional funding for his border wall. The bipartisan rebuke heightened a standoff between the executive and legislative branches of government and drew Trump’s first presidential veto on Friday, which Congress is not expected to override. The judiciary will then likely enter the fray and decide if Trump’s declaration is executive overreach. Both Texas senators, including Cornyn, voted against the legislative measure—a vote noted by Castro. When asked about the Cornyn campaign calling out Castro’s “Democrat-Socialist agenda,” Castro’s political adviser, Jones, responded: “Joaquin was not the one who just voted to allow the federal government to take hundreds of miles of land from Texans.”

The initiative by Castro came as the four-term congressman seems to be exerting significant political influence in the House for the first time as a member of the majority party. In addition to his role in the the constitutional debate, Castro is chairman of the Congressional Hispanic Caucus. He also serves on the House Committee on Intelligence and is vice chairman of the Committee on Foreign Affairs. He is also campaign chairman for his brother’s presidential run—a role that may change with the launch of a senate campaign.

Gilberto Ocañas, a longtime Democratic operative, said Cornyn’s vulnerabilities to a Castro campaign go hand in hand with Cornyn’s defense of Trump as the majority whip in the Senate—a leadership role that he left in December because of Republican term limits. “The most important thing is that he stands behind a president whose policies have hurt Texas—from attacking our largest trading partner in Mexico to attacking Hispanics,” Ocañas said. “Cornyn seems to lack the courage to represent Texas first.”

Cal Jillson, a political scientist at Southern Methodist University, said a Castro entry into the campaign “is going to juice things up.” He said the brothers’ political career has always been mutually beneficial. “They have both been quite careful and supportive of one another,” he said. Jack Martin, a longtime Texas Democratic operative, agreed. “They have always worked as a team. Their assets are transferable.”

One example of that at play is the timing of Thursday’s revelation that Castro has all but made up his mind to jump into the race, Sabato said. News of Castro’s likely entry into the Senate race may help mute some of the media coverage of O’Rourke’s entry into the presidential race.

A Castro entry into the race may also foreclose a political option for O’Rourke, Martin said. If O’Rourke’s presidential campaign were to flounder in an increasingly crowded Democratic field, O’Rourke still would have the option of dropping his bid at the national level and jumping into the Senate race. A Castro candidacy makes that option less attractive.

An active campaign by the Castro brothers also raises the possibility of increasing Hispanic voter turnout, Sabato said. “Hispanics are the key to changing Texas [purple].” But Martin cautioned that Joaquin Castro must run a multidimensional campaign against Cornyn. “He needs to remember that the Hispanic vote isn’t particularly where the big gains were made in the last election. That occurred in the suburbs and among African Americans.”

Already, with $5.8 million in the bank, Cornyn has amassed more campaign cash for next year than any other senator, according to Politico. Martin believes, however, that Joaquin Castro will have an easier time in the early part of the campaign raising money than his brother will in a crowded Democratic primary. More money would flow if Julián is able to secure his party’s nomination, but there is the potential that before that happens the two brothers may be in a position in which they are competing for the same pot of money.

Beyond the money, Martin cautioned that Cornyn has long been a formidable campaign opponent. Within two hours of a request for comment about a Castro candidacy, the Cornyn team began painting Castro as a socialist who didn’t care about Texans. “Joaquin Castro voted against Hurricane Harvey relief—twice,” the campaign wrote. “Representative Castro’s first vote against tax relief for Harvey victims was only weeks after it hit Texas. Five months later, he voted against $90 billion in disaster funds, including for Texans hit by Harvey.”

Then the campaign noted Castro’s support for a Green New Deal. “Representative Castro has cosponsored Democrat-Socialist Representative [Alexandria] Ocasio-Cortez’s Green New Deal resolution.”

Many, however, see Cornyn as vulnerable during a presidential election year when a controversial president is expected to be running for reelection. A February University of Texas/Texas Tribune poll showed Cornyn with an overall approval rating of 36 percent to 35 percent disapproval rating. Cruz had a much higher approval rating of 46 percent to 41 percent disapproval rating. Cruz still outpolled Cornyn among those who identify Republican by more than 20 percentage points. Cornyn has an approval rating among Republicans polled of 62 percent and a 14 percent disapproval rating. Cruz had an 83 percent approval rating among Republicans polled and a 7 percent disapproval rating.

“The reason Cornyn may be vulnerable is because the Republican party in Texas has changed dramatically,” Jillson said. “In 2014, the tea party candidates beat the establishment candidates two to one in the primaries.” Cruz, he said, is a tea party candidate. Cornyn is not. And while Cornyn’s more centrist views have served him well, Sabato added, the controversy that Cruz has created has only made Cruz’s base more loyal. That base is not as loyal to Cornyn.

An earlier version of this story incorrectly referred to Joaquin Castro as campaign manager for Julián Castro’s presidential run. Joaquin is Julián’s campaign chairman, while Maya Rupert is Julián’s campaign manager.