WASHINGTON — His hand chopping in the air, his voice stern and stalwart, he declared that it was time for the regional despot to go and warned of the consequences if he did not. With a commander in chief’s resolve, he vowed that the United States would do whatever it took to protect its own diplomats on the ground.

It was not the commander in chief but Senator Marco Rubio, the Florida Republican who nearly three years after losing his own bid for the presidency has become a lead policy architect and de facto spokesman in a daring and risky campaign involving the United States in the unrest that is now gripping Venezuela.

Through sheer force of will and a concerted effort to engage and educate President Trump, Mr. Rubio has made himself, in effect, a virtual secretary of state for Latin America, driving administration strategy and articulating it to the region from the Senate floor, as he did the other day, and every television camera he can find. Perhaps no other individual outside Venezuela has been more critical in challenging President Nicolás Maduro.

“He’s picked a battle he can’t win,” Mr. Rubio, 47, said of Mr. Maduro in an interview on Friday. “It’s just a matter of time. The only thing we don’t know is how long it will take — and whether it will be peaceful or bloody.”