“All of us, wherever you live, can acknowledge that if immigration is a problem it’s the best possible problem for this country to have,’’ Mr. O’Rourke said in his video announcement Thursday.

Mr. Trump, in turn, has taunted Mr. O’Rourke for turning a losing campaign into a launchpad for a presidential bid. “Hey, you’re supposed to win in order to run,’’ Mr. Trump has said.

But the Democratic primary could present unique challenges to Mr. O’Rourke.

It is an open question whether he will be able to scale up his skeletal organization and hand over control to the sort of political professionals he largely shunned in his Senate race. The lead-up to Mr. O’Rourke’s official announcement Thursday has been highly improvisational, in part because he was personally directing much of the planning.

On Wednesday night, he was texting supporters in early nominating states to share his plans and to tell them he would have advisers get in touch with them about his upcoming schedule. And for weeks, he has been meeting and talking on the telephone with a number of Democratic strategists to gauge their interest in working for him, finding encouragement but also a reluctance to move to El Paso, where he is planning to base his operations.

As late as last weekend, Mr. O’Rourke had still not settled on who would guide his campaign. He discussed the campaign manager job for 90 minutes with a Democratic strategist, Jennifer O’Malley Dillon, at the South by Southwest festival in Austin, but even on the eve of his announcement it was uncertain who would be at the helm of his organization.

Yet he enjoys the support of many of former President Barack Obama’s aides, some tacitly and others more full-throated, and he has relied on advice from a number of Mr. Obama’s strategists, including the 2008 campaign manager, David Plouffe. (Mr. Plouffe is not, however, planning to formally participate in the race on behalf of any candidate.)

But unlike Mr. Obama, who ran in a year when the Iraq war was the single overriding policy issue in the Democratic race, Mr. O’Rourke is seeking the presidency at a moment his party is lurching left across the board. He will be immediately under pressure to expand upon the sometimes-vague liberalism that has colored his public life.

“During a crisis, heroes come to the forefront because many of our basic human needs are threatened, including our need for certainty, meaning and purpose, self-esteem, and sense of belonging with others,” said Elaine Kinsella, a psychology professor at the University of Limerick in Ireland who has researched the role of heroes in society.

“Heroes help to fulfill, at least in part, some of these basic human needs,” she added.

The scientist-heroes emerging from the coronavirus crisis rarely have the obvious charisma of political leaders, but they show deep expertise and, sometimes, compassion.

In Italy, a nation ravaged by the virus more than any other in the world so far, Dr. Massimo Galli, the director the infectious diseases department at Luigi Sacco University Hospital in Milan, swapped his lab coat for a suit and accepted he “would be overexposed in the media” in order to set things straight, he told one talk show.

In Greece, which has so far been spared a major outbreak, everyone tunes in when Prof. Sotirios Tsiodras, a slender-framed, gray-haired man, addresses the nation every day at 6 p.m.

His delivery is flat, and he relies heavily on his notes as he updates the country on the latest figures of those confirmed sick, hospitalized or deceased. Occasionally, he offers practical advice, like a solution of four teaspoons of bleach per liter of water can be sprayed on surfaces for disinfection.

He approaches the mosque on foot, his weapon visible, and begins shooting at people at the entrance. What follows is a harrowing nearly two minutes of his firing on worshipers.

At one point the gunman exits the mosque and fires in both directions down the sidewalk before returning to his car for another gun — which, like the others, was inscribed with numbers, symbols or messages. When he re-enters the mosque, he shoots several bodies at close range.

After another few minutes, he returns to his vehicle and drives away.

“There wasn’t even time to aim, there was so many targets,” he says at one point, as the sirens of an emergency response vehicle blare in the background.

— MEGAN SPECIA and JASON BAILEY

A white nationalist manifesto

Before the shooting, someone appearing to be the gunman posted links to a white-nationalist manifesto on Twitter and 8chan, an online forum known for extremist right-wing discussions. The 8chan post included a link to what appeared to be the gunman’s Facebook page, where he said he would also broadcast live video of the attack.

The Twitter posts showed weapons covered in the names of past military generals and men who have recently carried out mass shootings.

In his manifesto, he identified himself as a 28-year-old man born in Australia and listed his white nationalist heroes.

Writing that he had purposely used guns to stir discord in the United States over the Second Amendment’s provision on the right to bear arms, he also declared himself a fascist. “For once, the person that will be called a fascist, is an actual fascist,” he wrote.

— DANIEL VICTOR and TIFFANY MAY

YouTube star ‘sickened’ by being cited in video

Felix Kjellberg, a polarizing YouTube celebrity known as PewDiePie, distanced himself from the attacks after the man who filmed himself shooting victims at a mosque encouraged viewers to “subscribe to PewDiePie” in a video livestream.

“I feel absolutely sickened having my name uttered by this person,” Mr. Kjellberg, a Swede, said on Twitter.

Mr. Kjellberg has courted controversy by performing anti-Semitic gestures, which he calls satirical, in his videos. He has a following of 89 million subscribers.

— TIFFANY MAY