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“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.