El senador Jeff Merkley, un incondicional silencioso de la izquierda, tiene un momento de quiebre en la frontera

WASHINGTON — It did not exactly make for riveting video this month when Senator Jeff Merkley, Democrat of Oregon, was turned away from a converted Walmart at the Texas border that is housing hundreds of migrant children who have been taken away from their parents by the Trump administration.

Shirt sleeves rolled up and cellphone in hand, Mr. Merkley spent the better part of half an hour waiting outside the shelter’s doors, talking calmly to the cameras until the police arrived, at which point the shelter’s supervisor emerged and asked him to leave. He politely complied.

But when the social-media savvy senator streamed the encounter on Facebook Live, it promptly went viral, setting in motion a heated debate that ultimately forced President Trump to reverse himself, and issue an executive order that migrant families be kept together when they are apprehended at the border.

Now the mild-mannered Mr. Merkley is having a breakout moment. A darling of the progressive movement, he has until now been an under-the-radar presence in Washington, overshadowed by his high-profile colleagues, Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts. Though a champion of same-sex rights and environmental causes, he has not been a legislative standout, and is known mostly as the only senator who endorsed Mr. Sanders over Hillary Clinton in the 2016 Democratic presidential primaries.

He went to the border, he said in an interview on Thursday, because he thought reports of children being separated from their families might be fake news: “I just couldn’t envision that the administration would actually take asylum seekers, those fleeing persecution, and deliberately inflict trauma on their children.”

It is undoubtedly not lost on him that the ensuing publicity is good for his career; Mr. Merkley also acknowledged that he was “exploring the possibility” of a 2020 presidential bid. (He has previously said he was “keeping the options open.”) Asked whether he would stay out of the race should Ms. Warren or Mr. Sanders become candidates, he leaned back in his chair and said, “Not necessarily.”

Mr. Merkley’s unsuccessful foray to the shelter, in Brownsville, has not been without controversy. While in Texas, he also visited a processing center at the border station in McAllen, and came back describing “hundreds of children locked up in cages there” — an assertion that helped earn him a “Three Pinocchios” rating in a fact-checking report by The Washington Post.

That rating was later downgraded to “Two Pinocchios” after lawmakers and journalists corroborated Mr. Merkley’s claim; the remaining Pinocchios involved his inaccurate assertion that outsiders are barred from visiting the shelter when visits are, in fact, allowed with two weeks’ notice. The White House, for its part, accused Mr. Merkley of “irresponsibly spreading blatant lies.”

Mr. Merkley, who has never met the president, is steamed about that. “I know I took a lot of heat for using that term, but that’s what they look like,” he said, referring to the “cages” comment.

In the interview, he also challenged Mr. Trump — whose wife, Melania, made an unannounced visit on Thursday to a children’s detention center in McAllen, while the administration made plans to house up to 20,000 unaccompanied migrant children on military bases — to make his own border trip.

“I think if you’re going to do a policy that injures people,” Mr. Merkley said, “you ought to have the guts to go see it firsthand.”

Tall and gray-haired at 61, Mr. Merkley is not, by his own admission, the kind of politician who makes audiences swoon.

“I’m not the person who comes to battle with, if you will, extensive charisma,” he said. “That’s not me. But I am determined.”

Indeed, if there is one word his colleagues use to describe Mr. Merkley, it is “persistent.” In trying to win support for the Equality Act, a bill that would amend the 1964 Civil Rights Act to protect gay, lesbian, bisexual and transgender people, Mr. Merkley badgered fellow Democrats and the independents who caucus with them until all but one of them signed on. That persistence extends to his personal life; he has also completed two Ironman triathlons while serving in the Senate.

“He’s like a dog with a bone,” said Senator Richard Blumenthal, Democrat of Connecticut. “He will just not let go.”

But some find that quality irksome, and Mr. Merkley’s border attention grab has clearly irritated at least one Republican. Senator Roy Blunt of Missouri at first grew quiet when asked about Mr. Merkley — a telltale sign among senators that they do not have something nice to say.

“Well,” Mr. Blunt finally allowed, “other people went to the border when Obama was president on our side, and were never allowed in, and it was never any news.”

The son of a mill worker and a homemaker, Mr. Merkley proudly wears his blue-collar roots; he is the first person in his family to graduate from college. (Stanford for his undergraduate degree, Princeton for a master’s in public policy.) He is a big do-it-yourselfer — aides call him “thrifty” — and has posted videos of himself on social media repairing his deck fixing his lawn mower at home in Oregon.

“He’s a beer and burgers guy,” said Stephanie Taylor, a co-founder of the Progressive Change Campaign Committee, a liberal group that has raised money for Mr. Merkley.

A one-time intern for Senator Mark O. Hatfield, a moderate Republican, Mr. Merkley came up in politics as a State House representative in Oregon. A close friend and adviser, Robert Stoll, remembers him as “a very quiet, very studious sort of guy, and a little geeky.” In 2007, after leading an effort to flip the House from Republican to Democratic control, he was elected speaker.

That same year, he set out to take on Gordon Smith, a senator at the time and another popular moderate Republican. “We said, ‘You have a 20 percent chance of winning,’” Mr. Stoll recalled. “He said, ‘That good? I’m running.’”

Mr. Merkley won that race, swept in on Barack Obama’s presidential coattails. Once in the Senate, he championed banking overhaul and led a successful push to change Senate rules so that Mr. Obama’s judicial nominees could be more easily confirmed. More recently, he has endeared himself to the Trump resistance by hosting meetings every two weeks in his office where outside groups like Indivisible and Move On swap strategy with senators.

On Sunday, Mr. Merkley again traveled to Texas, this time leading a delegation that included Senator Chris Van Hollen, Democrat of Maryland, and several House members. They made five stops; what he saw there haunts him.

At a legal port of entry, Mr. Merkley said, he witnessed American border guards “blocking people seeking asylum from getting to the U.S.” At a detention center, he said, he met with 10 women whose children had been taken away at the border; they had no access to legal representation, he said.

At a border patrol center, he met a woman from Honduras who told him she had fled, at eight months pregnant, after members of a drug cartel threatened to kill her if she did not pay a loan to a local bank. She gave birth during her journey north. Under new, more restrictive rules imposed by the Trump administration, Mr. Merkley said, the woman is likely not eligible for asylum in the United States.

As Congress wrestles with whether to pass legislation addressing the border crisis, Mr. Merkley is skeptical; he says legislative debates in Congress are “like stepping in quicksand — it sucks you down and you can’t move.”

He calls Mr. Trump’s executive order “a travesty,” because it allows migrant families to be detained indefinitely — a policy he has nicknamed “handcuffs for all.” His next step is to have Democrats convene a “shadow hearing” on the topic, perhaps as soon as next week. He is hoping to have migrant families testify.