Editor's note: O'Rourke did indeed announce a Senate run on March 31.

WASHINGTON — U.S. Rep. Beto O'Rourke will launch a campaign to challenge U.S. Sen. Ted Cruz in 2018 on Friday, according to a colleague with direct knowledge of the El Paso Democrat's thinking.

O'Rourke's campaign team sent an email to supporters Wednesday morning revealing that "a big announcement" was coming Friday.

"Together, we can do something really big, and really powerful for the state of Texas — and for this country," the campaign wrote. "Congressman Beto O'Rourke has a big announcement to make on Friday." 

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O'Rourke has spent the past several weeks traveling the state and has said in recent weeks that he is likely to launch a bid for U.S. Senate. Multiple Democratic leadership sources on Capitol Hill have been operating under the assumption that an O'Rourke Senate campaign is inevitable.

A Cruz spokesperson declined to comment on the announcement.

If O'Rourke does enter the race, he could face a competitive primary; U.S. Rep. Joaquin Castro of San Antonio is considering a bid. 

The speculation Wednesday over O'Rourke prompted Castro's team to release a statement on his future political plans.

“It’s no secret that Joaquin is heavily weighing a Senate run, and he will continue to have those discussions with his family, friends and supporters across Texas," said Matthew Jones, Castro's political director, in a statement. "He plans to make his decision in the coming weeks.”

Jones added that Castro "has always led the charge on tough fights, so one way or another, he will be on the front lines of the 2018 midterm elections doing everything he can to continue to hold Republicans accountable.”

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A Castro bid is no sure thing; he has a rising profile in Congress, he's on the leadership track in the House of Representatives and he's deeply involved in the investigations into whether Russia interfered with the 2016 elections. 

O'Rourke, by contrast, has made plain since his insurgent initial race for the House in 2012 that he would impose term limits on himself. A logical next step would be a race for Senate. 

There is an additional potential outlier in this mix: Former George W. Bush strategist Matthew Dowd, an on-air commentator, is considering an independent run for Senate.

No matter who the Democratic nominee is, a race against Cruz will be an uphill slog. The state is still firmly conservative, and Texas has not elected a Democrat statewide since 1994. O'Rourke is still relatively unknown in Washington and in Texas, and El Paso has never produced a statewide elected official. 

Cruz, meanwhile, appears to be steadying his political ship after a rough post-presidential campaign stretch that included being booed off the stage at the Republican National Convention. The incumbent kept many of his presidential campaign staffers within his sphere, and he has built a reputation for running well-funded and tactically savvy campaigns.

"Well, I like Beto O'Rourke ... but if he thinks he can beat Ted Cruz, I think he's sadly mistaken," said Cruz's fellow Texan in the Senate, Republican John Cornyn, when asked about O'Rourke's likely candidacy Wednesday. "We'll continue to work together where we can, but I don't think he's got a shot." 

From a national perspective, the U.S. Senate race in Texas is an afterthought. The Democratic Party is preoccupied with protecting 10 incumbents who represent states President Donald Trump carried in 2016. Hardly anyone in state or national politics considers it realistic that the national party will heavily invest in this race.

But the race could still garner attention. O'Rourke has a knack for harnessing the Internet. Last year, he was one of the members who livestreamed a Democratic protest on the floor of the House chamber, and he shared a cult following with U.S. Rep. Will Hurd, R-Helotes, on a recent bipartisan cross-country road trip. 

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And while it's not yet clear what 2018 will bring, but the early months of the Trump presidency have been turbulent for the president and congressional Republicans, who last week admitted they couldn't get their party together to support a proposed overhaul of President Obama's signature health care bill. 

O'Rourke first floated the notion of a Senate campaign to The Texas Tribune just before Election Day last fall. 

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