Editor's note: This story has been updated throughout.

WASHINGTON — U.S. Rep. Joaquin Castro will not run for Senate after all.

After repeated public signals that he was "all but certain" to challenge Republican U.S. Sen. John Cornyn, Castro, a San Antonio Democrat, has opted to run for reelection rather than pursue what would likely have been a bruising political battle.

The San Antonio Express-News was the first to report the news Wednesday.

But Castro's decision first came to light earlier in the day, when he told a reporter that he's "gonna pass" on the race to unseat Cornyn. That conversation with Castro was overheard by other media outlets on Cornyn's weekly conference call with the Texas media. The reporter apparently had not muted his line on the Cornyn call and was talking with Castro on a different line.

Castro told the reporter that their conversation was "off the record," which refers to an agreement between a reporter and a source to not publish what the source says. But other journalists who heard the conversation are not bound by any such agreement.

All eyes now shift to Air Force veteran MJ Hegar, a 2018 U.S. House candidate who lost her bid to unseat U.S. Rep. John Carter year. She announced her Senate run last week. While unsuccessful in her congressional run, Hegar built a strong fundraising operation and exceeded expectations in that bid. She is likely to have the support of EMILY's List, an organization that works to elect female Democrats who support abortion rights.

For now, this race is Cornyn's to lose. He has several million dollars in his campaign account and is unlikely to face a serious primary challenge. But national Democrats are cautiously sizing up Texas, trying to determine if they are willing to invest enough to compete for a state with as many media markets as Texas.

But there is a great deal of activity down-ballot — Democrats are competing for both U.S. House and state legislative races. And former U.S. Rep. Beto O'Rourke's closer-than-expected 2.6 point margin against U.S. Sen. Ted Cruz last year has done much to lift Democratic hopes.

Cornyn has sought to sow division in the Democratic primary, portraying Hegar as the choice of Senate Minority Leader Chuck Schumer, D-N.Y. Cornyn continued to do that during his conference call Wednesday, even as a reporter's exchange with Castro was still unfolding for all to hear.

Castro could run, Cornyn said, "but maybe on second thought, he's decided to stand down and just accede to Mr. Schumer's hand-selected candidate, MJ Hegar."

Cornyn's campaign further pushed the narrative in a statement on Castro's decision, saying, "Shame on Chuck Schumer and DC Democrats for forcing a high-profile Hispanic leader out of the Senate race."

Castro previously brushed off the Schumer-related speculation as "political gossip."

Hegar released a statement Wednesday praising Castro for "his commitment to public service" and "his strong leadership."

"I am laser-focused on our shared goal of defeating Senator Cornyn next November," she said.

Hegar is one of four Democrats who have announced they are running against Cornyn. The others are Michael Cooper, Sema Hernandez and Adrian Ocegueda.

Houston City Councilwoman Amanda Edwards has also said she is considering a run for the seat, and state Sen. Royce West of Dallas has been discussed as a potential candidate. Shortly after Castro announced his decision Wednesday, West told the Tribune that he is focused on the current legislative session and its two big issues: school finance and property tax reform.