WASHINGTON — U.S. Rep Filemon Vela’s open letter to Donald Trump, that missive challenging the presumptive Republican presidential nominee to ram a theoretical border wall up his ass, is already the stuff of Capitol Hill legend.

Practically every staffer and lawmaker at the Capitol has read it and talked about it. Even a day later, congressional staffers were still sheepishly laughing at how the Democratic Brownsville congressman took it to Donald Trump.

Vela's letter almost seemed to come out of nowhere. Sure, there are plenty of liberal congressional firebrands who use incendiary prose to secure television bookings or raise money.

But thus far into his brief career, Vela has not fit that profile.

“I was surprised, and frankly glad,” said U.S. Rep. Gene Green, D-Houston, of the letter. “That was not the Fil Vela I know. He’s a lawyer. We typically make legal arguments, but I think it shows the frustration in his district and even mine about what Donald Trump has been saying.”

Since Vela came to Congress in 2013, he’s earned a reputation as a quiet workhorse who raises money for colleagues and eagerly hosts powerful Democrats in his South Texas district. He’s shown them the 34th district’s beaches, the King Ranch and the border, and he’s urged his colleagues to send aid and infrastructure.

Beyond South Texas, he’s an emerging power broker within the Texas Democratic delegation. He's racked up many favors, but so far no one can think of a time when he called them in.

Known on Capitol Hill by his nickname “Fil,” he is an enigma and a head scratcher. And with this fiery burst of a letter, the question for many this week was, “What does Fil want?”

“I like obscurity,” Vela said with a laugh during an interview Tuesday at the U.S. Capitol.

This week, he’s anything but. Suddenly the back-bencher was juggling cable news bookings.

“ That was not the Fil Vela I know. He’s a lawyer. We typically make legal arguments, but I think it shows the frustration in his district and even mine about what Donald Trump has been saying. ” — U.S. Rep. Gene Green, D-Houston

Vela said the roots of the letter were his constituents’ concerns about Trump’s rhetoric, but the last straw was Trump’s Cinco de Mayo tweet with a taco salad and recent comments about whether a Hispanic judge can be partial in litigation involving a Trump educational enterprise.

“There is no connection between him and people of Hispanic origin, and then these comments about the judge as the icing on the cake,” Vela said. “I mean, who would ever say that?”

The congressman said he composed the letter over a several days late last week while driving from Texas to Washington. There were many drafts, and along the way he stopped off at the Lorraine Hotel in Memphis, the site of Dr. Martin Luther King’s assassination.

"Mr. Trump, you’re a racist and you can take your border wall and shove it up your ass," he wrote.

Vela said the times called for the breach of congressional decorum.

“I would never use that word in a public setting, but Trump’s tone over the last three months has just taken us to a point where the only way you’re going to be able to respond to him is to speak in his language,” he said. “I’ve never made a public statement and used that language, and hopefully I never will again.”

Most Democrats interviewed agreed that Trump is a singularly provoking force in discourse.

“That’s not what we do,” said Green, a close friend of Vela’s. “[But] it’s not like we’ve never heard that word. Normally you don’t hear it from members of Congress, but I think it shows the level of intensity of what’s happening on the Republican side.

“I don’t think anybody is happy we got to this stage, but the only way you can deal with it is to fight fire with fire, and it comes from someone who’s not a screamer and not one of our folks who gets up everyday and speaks on the floor of the House.”

Vela came to Congress as member of the class of 2012, a good year for their party. As a result, he is a member of an enormous Democratic class jammed with impatient up-and-comers. Many have their eyes on leadership or the U.S. Senate.

“I have always enjoyed working from the shadows,” Vela said of his political involvement prior to Congress.

Before the letter, he was mostly known around Washington as the Texan who invites his colleagues down to his district and as a loyal fundraiser for other Democrats. By his own estimate, he’s brought over 60 members down to South Texas to show them the local needs.

“I enjoy the politics of the office,” he said. “So, I think relationship building is important for the district. I think it’s really beneficial.”

And as a freshman, he also earned some notice when he quit the Congressional Hispanic Caucus over an immigration reform policy dispute. He later rejoined the group.

“ I’ve never made a public statement and used that language, and hopefully I never will again. ” — U.S. Rep. Filemon Vela, D-Brownsville

More recently, he took the California Democratic Party to task for favoring another Democrat against U.S. Rep. Loretta Sanchez in the state’s Senate race.

But it’s back home in Texas where his power is felt.

Late last year, Green found himself in the fight of his life. Former Harris County Sheriff Adrian Garcia challenged Green in the 29th District’s Democratic primary, arguing that the predominantly Hispanic seat needed Hispanic representation.

Vela instantly offered Green, an Anglo, racial cover. But he went further: He loaned Green his billingual spokesman, Jose Borjon, for the duration of that campaign.

Moses Mercado, an influential Washington lobbyist who is also close to the congressman, told the Tribune that Vela was crucial to Green’s re-election because he helped the older man — who had not run a serious race since the mid-1990s — get up to speed on social media and evolving campaign technology.

A few months later, Vela played a more indirect role in the open-seat race to replace retiring U.S. Rep. Ruben Hinojosa.

Members of Congress generally avoid endorsing candidates in open primaries for fear of picking a loser. But high-ranking Democrats from across the country flocked to support attorney Vicente Gonzalez in the party primary.

Democrats in those circles say they picked Gonzalez because Vela vouched for him, although his support fell short of a full-blown endorsement.

The fight he picked with Trump might well be Vela’s breakthrough moment in national politics. And yet, his ambitions remain unclear. House Democratic staffers speculate he might one day lead the Hispanic Caucus or run for leadership. He has the chits to make the case.

“He’s very smart about who the key people are, which is interesting,” Mercado said. “He raises all this money, and not once has he said ‘I’m thinking about doing this, will you support me?’”

“He just does it, which is a breath a fresh air.”

But no one really knows, even Vela.

“I haven’t really thought about where I’m going to be five years from now,” he said. “We’ll just have to wait and see."

Julián Aguilar contributed to this report.