Updated at 4:45 p.m.: Revised throughout.

WASHINGTON — San Antonio Rep. Joaquin Castro on Wednesday revealed that he will not mount a bid for U.S. Senate in 2020, marking the second time in two election cycles he’s passed on high-profile opportunities to run for statewide office in Texas.

Castro — whose twin brother, Julián, is running for president — would’ve entered the Senate fray as a well-known political commodity, one who wields growing influence in Congress and who has long been marked for higher office by Democratic power players.

His stunning decision instead leaves Air Force veteran MJ Hegar as the only well-known Democrat so far in the race to take on Republican Sen. John Cornyn.

It also does little to change the narrative that Castro is cautious to a fault — particularly after he teased for months that he might step up to the challenge this time around and just last week stoked speculation by having his campaign taunt Cornyn as President Donald Trump’s “enabler.”

But the congressman would’ve had to relinquish his House seat — and all the clout he’s built up — to make an uphill run to topple Cornyn. So he offered a similar justification to what he provided two years ago, when he said he could best serve Texas by staying in the House.

"Right now, I'm going to focus on my work in the House of Representatives," he said Wednesday to Hearst Newspapers, which includes his hometown newspaper, the San Antonio Express-News. "I've been doing what I feel is important and meaningful work here."

Castro's choice was first reported by the Express-News. But it was unveiled earlier in the most bizarre manner -- inadvertently on a conference call that Cornyn, of all people, was holding with Texas reporters.

A reporter participating in the Cornyn call didn't mute his line when Castro called on a different line to discuss, off the record, the looming decision. While the conversation was eventually cut off as Cornyn proceeded to take questions, Castro could be overheard telling the reporter, "I'm gonna pass."

The Express-News' story published about an hour later.

San Antonio Rep. Joaquin Castro announced Wednesday that he would not run for Senate, telling Hearst Newspapers that he's "going to focus on my work in the House of Representatives." (Eric Gay / The Associated Press)

Texas still considered a battleground

While Castro’s absence takes some shine off of the 2020 throwdown in Texas, it doesn’t negate the state’s newfound battleground status.

It’s well-known that a Democrat has not won statewide office in Texas since 1994. The underlying challenge has gone deeper as party leaders have struggled in the past to find top-flight candidates to wage what often turned into embarrassing slogs for senator or governor.

Hegar alone could avoid that kind of quagmire, particularly if the Cornyn campaign's response to date is any guide.

Team Cornyn, which has been preparing for a tough re-election fight, has been jabbing at Hegar as a real threat ever since she announced her bid last week. Cornyn, who remains the favorite, did the same earlier this week when asked to size up the prospect that Castro may not enter the competition.

"It means Chuck Schumer would've cleared the competition out," he said, referring to rumors that the Senate Minority Leader favored Hegar's candidacy. "I presume that comes with promises of a lot of money coming from out-of-state donors to try to change Texas.

"And we’re not going to let that happen.”

Cornyn campaign manager John Jackson went further on Wednesday, blasting Schumer and "D.C. Democrats for forcing a high-profile Hispanic leader out of the Senate race."

"Chuck Schumer recruited and picked MJ 'Hollywood' Hegar instead, pushing Castro aside and ignoring the Texas Latino grassroots," he said. "Hollywood Hegar may have the support of the New York and California elites, but she has none in Texas."

Texas Sen. John Cornyn remains the favorite to win re-election in 2020, though Texas Democrats are eager to unseat him. (Eric Gay / The Associated Press)

Castro’s decision staves off an unfamiliar test for Texas Democrats, who’ve been looking to build upon the scare former El Paso Rep. Beto O’Rourke gave last year to GOP Sen. Ted Cruz.

A rough-and-tumble primary — which remains possible, depending on who else gets in — could battle-test the ultimate nominee. But it could also divide loyalties, strain resources and leave self-inflicted wounds for Cornyn to exploit in the general election.

Castro's complications

A Castro bid would’ve come with its own complications.

The San Antonian has little in his campaign war chest, and he and his brother likely would’ve been courting the same donors. He had also already frustrated some Democratic brass by drawing out his decision-making process, particularly after doing a similar dance two years ago.

But there’s no question Castro’s duck will disappoint some Democrats, including those who’ve been pining for a competitive field.

“Very clearly contested primaries are good for the party,” said Brownsville Rep. Filemon Vela, who had said before Castro’s announcement that his longtime friend was “our party’s best candidate against Sen. Cornyn.”

Castro's choice to pass on a Senate campaign cemented another critical juncture in a political career that's come into its own in the Democrat-run House.

The 44-year-old is chairman of the Congressional Hispanic Caucus, an influential group on Capitol Hill. He sits on powerful congressional panels like the House intelligence committee. And he's taken on some hefty legislative tasks in recent months.

Earlier this year, for instance, Castro authored a resolution of disapproval against Trump's national emergency at the U.S-Mexico border.

The unprecedented rebuke passed both the House and Senate before being vetoed by Trump. Neither Cornyn nor Cruz supported the effort to block the national emergency -- a fact that Castro used at the time as a political cudgel against the Republicans.

"Fifty-nine senators stood up to the president today," Castro said then. "Unfortunately in Texas, that did not include either of our senators ... who showed weakness today, who showed timidity, who betrayed the people of Texas in favor of Donald Trump."

That kind of profile made Castro among the most obvious candidates to follow in the footsteps of O'Rourke, who's now running for president. But it also likely explains his decision to pass.

Castro has spent the majority of his career serving in the minority party, whether in Congress or in the state Legislature. So he's just now getting to enjoy the fruits of being in the majority. Other statewide opportunities remain possible down the road, including a gubernatorial race in 2022.

"If and when I run for another office, it is likely to be something that takes me back home to Texas," he told the Express-News.

A Castro Senate bid would've also faced key obstacles.

Those hurdles would've begun with Hegar, a Round Rock Democrat who last year nearly knocked off longtime GOP Rep. John Carter and who has already proven her skill at two important tasks needed to succeed in an expansive state like Texas: fundraising and social media.

Hegar said Wednesday that she has "tremendous respect for Castro," before explaining that she's "laser-focused on our shared goal of defeating Senator Cornyn next November."

"Texans deserve a new senator who reflects our values of strength, courage, and independence -- someone who will always do what's right for our state and put Texas first," she said in a news release.

Castro is not exactly a fundraising juggernaut, reporting a $88,000 war chest at the end of March that paled into comparison to Cornyn's $7.4 million cash on hand.

And there remains a chance that other notable Democrats could join the race, with state Sen. Royce West of Dallas and Houston City Council member Amanda Edwards among those rumored to have interest in taking on the longtime Republican senator.

Eyes on Texas race

No matter the final field, the battle is expected to draw lots of attention in Texas and across the U.S.

The Texas Democratic Party has already launched a multimillion-dollar "war room" to take down Cornyn. Castro told the Express-News that he was impressed by Hegar, adding that he'll "do everything I can to help our Democratic nominee win."

The Democratic Senatorial Campaign Committee is also keeping close tabs.

"Senator Cornyn is a career politician who has spent nearly two decades in Washington taking orders from Mitch McConnell and the special interests when he should have taken them on," said DSCC spokesman Stewart Boss, referring the Senate Majority Leader.

Cornyn, meanwhile, has activated his re-election campaign much earlier than usual, warning Republicans that they can't become complacent. The Republican Senatorial Campaign Committee is also already active, zeroing in on Hegar.

"Grassroots progressives in Texas won't take kindly to a New York liberal like Chuck Schumer clearing the field in favor of Hollywood Hegar," said NRSC spokesman Jesse Hunt.