After more than half a century as an operatic powerhouse, Placido Domingo might easily take success for granted. Instead, each time he storms onstage as the ruthless ruler in Verdi’s “Nabucco,” he feels he has “more than ever to prove.”

“Because there are no excuses right now,” Domingo said. “I have made a long and beautiful career, and if I go out there it’s because I’m doing it well. Otherwise people will say, ‘Why is he doing it,’ you know?”

“So there is always a reason to have butterflies,” he added, in an interview at the Metropolitan Opera.

Domingo, who turns 76 this month, is enjoying a triumph in Verdi’s third opera, which tells the story of the Babylonian tyrant Nebuchadnezzar and his sacking of the temple of Jerusalem. The final performance will be broadcast to movie theaters worldwide live in HD on Saturday.

Originally a tenor, Domingo began turning to baritone roles in 2009 as the high notes became elusive. Although his sound lacks the weight and colors of a true baritone, audiences still are eager to see him. His “Nabucco” performances at the Met, like most of his appearances, have sold out.

“I think it’s a perfect role,” Domingo said, because of the variety of vocal and dramatic challenges. “In the first scene, honestly, it’s very difficult for the public to hear the baritone all the time. Because he’s singing with the whole chorus, but you have to come out strong.

“In the second act you have the chance first with sextet, and then the big sing with the craziness. And then comes the bel canto, the duet, and it continues with the aria and cabaletta.”

WALK LIKE THE ANIMALS

“The craziness” Domingo refers to is based on a biblical account of Nebuchadnezzar temporarily losing his sanity and living in the wild. Audiences may be startled to see Domingo at one point get down and crawl on all fours.

“He was kind of confused ... he was walking on four legs like animals,” Domingo said, “and that’s the reason I try to do it in the production.”

He also sings an entire aria — a plea to god for forgiveness — lying prone on his stomach, not the easiest feat for even a young singer.

“In one rehearsal I was just feeling ... all of a sudden, I just went and lay down and I tried it,” Domingo said. “And I realize that I can sing it this way, and it will be like a prayer. I’m totally lost, god forgive me.”

ENCORE, ENCORE?

The most famous number in the score is the stirring “Va, pensiero,” sung by a chorus of Hebrew slaves in Act 3. In Verdi’s day it became a rallying cry for Italian patriots seeking to free their country from Austrian rule.

James Levine, who is conducting the current performances, often has the chorus sing it twice, but that’s not guaranteed. “Like all encores, it sort of depends on audience response,” said Met spokesman Sam Neuman. So expect an encore during Saturday’s HD — but only if the applause in the house is sufficiently prolonged.

WHERE TO SEE IT

The HD broadcast of “Nabucco,” also starring soprano Liudmyla Monastyrska, mezzo Jamie Barton, tenor Russell Thomas and bass Dmitry Belosselskiy, will be shown starting at 12:55 p.m. on Saturday. A list of theaters can be found at the Met’s website: http://www.metopera.org/Season/In-Cinemas/Theater-Finder. Participating local theaters include Regal Stonington 10 and the Katharine Hepburn Cultural Arts Center in Old Saybrook.

In the U.S., it will be repeated on Wednesday, Jan. 11, at 6:30 p.m.