Beto O’Rourke has driven alone across the plains, straining to find his emotional bearings — or at least a Pancake House in Liberal, Kan. He has collected an “El Pasoan of the Year” award, before a modest audience, and smiled coyly through an interview with Oprah Winfrey, before a bigger one.

He has surfaced on college campuses, to listen to students; at a Metallica concert, to listen to Metallica; and at the premiere of a documentary about his star-making Senate run in Texas at the South by Southwest festival, to listen to himself on screen.

He has journaled extensively.

“We’re in this together, like it or not,” Mr. O’Rourke wrote, summarizing the lessons of his recent solo travels in one of several stream-of-consciousness online posts. “The alternative is to be in this apart, and that would be hell.”

As the Democratic presidential field takes final shape, Mr. O’Rourke seems inclined to be in this, according to interviews with people who have spoken to him and other top Democrats. He says he has made a decision about whether to run and could announce it as early as this week, unsettling prospective rival campaigns that consider Mr. O’Rourke a credible threat.