Castro-Cornyn matchup in 2020?

U.S. Rep. Joaquin Castro, D-San Antonio, coming off the biggest week of his political career, is considering challenging U.S. Sen. John Cornyn in 2020.

The surprising news came on the heels of reports that former U.S. Rep. Beto O'Rourke of El Paso is apparently taking a pass on the Senate race to join a Democratic presidential field that already includes Castro's twin brother, Julián.

It was Julián Castro who, on the campaign trail in Las Vegas, publicly broached the subject, telling The Associated Press late Thursday that Joaquin was thinking about taking on Cornyn, who is seeking a fourth term.

“I think he’d beat him," Julián Castro said. "My brother would win."

On Friday, Matt Jones, a campaign adviser to Joaquin Castro, confirmed that "Congressman Castro will seriously consider running for Senate in 2020."

"Right now, he's focused on protecting Texans — and all Americans — from the most consequential challenge to our constitutional separation of powers that we have seen in a generation," Jones said. "He will not stand by while the president attempts to unilaterally strip Texans of their land to build a wall in a manner that most Americans, especially Texans, disagree with."

Joaquin Castro spearheaded the successful effort by House Democrats to pass a resolution he drafted to block President Donald Trump's emergency declaration, which Trump issued to secure border wall funds that Congress had denied him.

“This is the most consequential vote we will take in a generation on the balance of powers between the legislative and executive branches of government,” Joaquin Castro said before the House voted 245-182 in favor of his resolution on Tuesday.

The measure still has to pass the Senate, which is possible, and survive a certain presidential veto, which might be virtually impossible. But it has already succeeded as an effective political response to the president.

A Joaquin Castro Senate candidacy would be an answered prayer for Texas Democrats as it has become clearer in recent days that O'Rourke, who narrowly lost a Senate challenge to U.S. Sen. Ted Cruz in November, was more interested in running for president than running for Senate again. On Wednesday, O'Rourke said he and his wife, Amy, had made a decision about their plans and were "excited to share it with everyone soon."

"It’s clear that Democrats need a top tier candidate to run against Cornyn. They can’t just hope for Trump to drag Cornyn down so far that any Democrat can win," Rice University political scientist Mark Jones said. "Right now, with Beto opting out, and Julián running for president, the best player that Democrats have on the bench is Joaquin Castro, whose stock has risen substantially even over the last week with his border wall repeal resolution."

The results of a Quinnipiac University poll released Thursday show O'Rourke in a dead heat with Cornyn in a trial 2020 matchup. According to the poll, O'Rourke, after his much-watched 2018 Senate race, is far better known to Texas voters than Cornyn, who was a state Supreme Court justice and Texas attorney general before being elected to the Senate in 1992.

The Quinnipiac poll did not ask about a potential Joaquin Castro Senate candidacy, but it did put O'Rourke, Julián Castro and other Democratic candidates head-to-head with Trump in Texas and found Trump in a virtual tie with O'Rourke, 47 percent to 46 percent, and Trump leading Julián Castro, 46 to 41 percent.

Those numbers are bolstered by a very high-profile Senate campaign in which O'Rourke raised more money than any U.S. Senate candidate in history, and before either Castro has run a statewide race.

"We’re going to be prepared for whoever decides to run and whoever is nominated for the Senate by the Democrats in 2020," Cornyn told Texas reporters Wednesday.

Many Texas Democrats have looked forward to the day that one of the Castro twins would run for statewide office. Joaquin Castro thought about running against Cruz in 2018, at a time when he was far better known than O'Rourke, but ultimately decided to cede the turf to his generational colleague — Castro is 44, O'Rourke is 46, and both entered Congress in 2013. O'Rourke surrendered his seat to run for Senate.

Even if it opens up the Senate opportunity for Joaquin Castro, an O'Rourke presidential candidacy would crowd Julián Castro's standing as the sole Texan in the Democratic contest, and as a candidate with strong generational appeal.

The Castro twins have pursued parallel political careers, but Julián Castro, born a minute earlier, has been first among equals, serving as mayor of San Antonio and secretary of housing and urban development under President Barack Obama. He delivered the keynote address at the 2012 Democratic National Convention, was considered for vice president by Hillary Clinton in 2016, has written a memoir, and is now running for president while his twin brother remains in the U.S. House.

But in the less than two months since Julián Castro launched his bid for president, it is Joaquin who has had the higher political profile, punctuated by this week's victory on the House resolution. He was elected chairman of the Congressional Hispanic Caucus in the new Congress, and was elected vice chair of the House Foreign Affairs Committee, as well as serving on the Education and Labor and House Intelligence committees. He has been integrally involved in the investigation into Russian meddling in the 2016 presidential campaign and potential Russian collusion by Trump and his campaign, and is a frequent cable news presence.

In his remarks at his brother's presidential announcement in San Antonio in January, Joaquin Castro wondered whether he should grow a beard now that his brother was running for president. He's done that, making him more easily physically distinguishable from his brother.

Joaquin Castro is chairman of his brother's presidential campaign. A Senate run would not only mean he would have to sacrifice his up-and-coming career in the House, but it also would distract from his ability to direct his brother's campaign and would put a financial strain on shared donors.

However, the campaigns could have a synergy that might prove helpful in Texas' important March 3 primary — and if it came to that, in the general election.

And twins from Texas simultaneously running for president and the Senate is a story that would draw the kind of national attention that Julián Castro's campaign could certainty use and could help draw national money to a Joaquin Castro Senate campaign.