His success would be all the more remarkable considering the scrappy operation Mr. Guaidó and his allies have mounted, seemingly overnight, to take down a heavily armed authoritarian government whose leaders stand accused of turning the state into a vast criminal enterprise.

As he tries to rally domestic and international backing, Mr. Guaidó has been crisscrossing Caracas with minimal security, at times weaving through traffic on the back of a motorcycle. He fields the calls from world leaders, as well as diplomats and Venezuelan politicians, on a cellphone that gets a torrent of text messages, and his phone’s battery seems always on the verge of dying.

If his sudden rise has been overwhelming, Mr. Guaidó shows no sign of feeling frazzled. His demeanor is calm as a volunteer team of aides dart around him frantically. The only outward sign that this tumultuous new life is getting to Mr. Guaidó is the hoarseness in his voice. At night, when he stops taking calls and visits, Mr. Guaidó said, he has no trouble falling asleep.

“I sleep placidly,” Mr. Guaidó said in an interview as his car stealthily moved through Caracas traffic, adding that he had learned to sleep soundly under trying conditions when he and other opposition leaders staged a 15-day hunger strike in 2015, sleeping on cots on the street, to pressure the government to set a date for legislative elections. “It’s an advantage.”

But Mr. Guaidó realizes the path he’s on is full of hazards.

Opposition leaders had hoped that when Mr. Guaidó proclaimed himself the country’s legitimate leader at a huge street rally, the chiefs of the armed forces would turn on Mr. Maduro in short order. While there have been a handful of high-profile defections, the top military brass has publicly rallied behind Mr. Maduro, which may portend a protracted standoff.