The Avengers unite against Ultron in the film “Avengers: Age of Ultron.” Marvel Studios

There was never any doubt that Disney's upcoming "Avengers: Infinity War" would rank among the biggest box office hits of all time. Simply by virtue of tying together a decade's worth of blockbusters, its place in the pantheon of movies appears all but assured. But after a surprisingly strong showing from the last few Marvel Studios movies, Hollywood prognosticators now say the tent-pole has a shot at posting the best U.S. opening of all time. The latest box office tracking for "Infinity War" puts its opening weekend haul at $190 million to $235 million. That's short of the $248 million "Star Wars: The Force Awakens" earned, but analysts say several factors could produce a box office surprise.

Once a movie starts tracking for a $150 million-plus opening, it becomes harder to forecast the exact figure, analysts said. The difficulty is compounded by the fact that no studio has ever done what Disney is about to do: Tie together dozens of characters developed over the course of 10 years and 18 movies.

'Uncharted territory'

"Infinity War" unites the Avengers and the "Guardians of the Galaxy" on screen for the first time, and further weaves in new characters, like "Doctor Strange" and "Spider-Man" into the Marvel Universe. It also features super-villain Thanos, who has lurked in the shadows of brief post-credit scenes and cameos for the last six years. "We're in uncharted territory here. We've never seen this exact combination of characters coming together," said Paul Dergarabedian, senior media analyst at comScore. "It's almost impossible to track at this point." What's clear is "Infinity War" is generating a huge volume of conversation on social media. It's now the second most talked about movie of all time, trailing only "Captain America: Civil War," according to comScore's PreAct analytics tool.

"Pre-release tracking and social media trends are exploding. Very few films have ever generated this kind of intense, widespread intrigue," said Shawn Robbins, chief analyst at BoxOffice.com, which currently forecasts "Infinity War" will earn open to $235 million to $255 million. The movie has already raked in $50 million in pre-sales, according to data seen by the Wall Street Journal. Another X-factor is the extraordinary winning streak Disney-owned Marvel Studios has been on since last year. "We didn't know 'Guardians of the Galaxy Vol. 2' was going to be a huge hit," said Dergarabedian. "We didn't know that 'Thor: Ragnarok' was going to hit the reset button on the Thor franchise. And of course we didn't know that 'Black Panther' would become one of the top grossing films of all time." "Black Panther" smashed expectations to become the 10th highest grossing film of all time, and one of only five movies to top $200 million in its North American debut.

Lupita Nyong'o in "Black Panther." Source: Disney

For many movie-goers, "Infinity War" essentially amounts to "Black Panther 1.5," said Jeff Bock, senior box office analyst at entertainment research firm Exhibitor Relations. "If just a fraction of those people show up to watch the further exploits of Black Panther and friends, who are featured prominently in this film, we could be looking at the number one opener of all time," he added. Bock also expects fans to pack theaters, because it could be their last chance to see their favorite heroes on the big screen. Many of Marvel's top-billed talent are at the end of their contracts, and speculation is running wild that the scriptwriters will kill off some major characters. "We're going to lose some superheroes here, and that doesn't always happen," said Bock. The loss of Chris Evans as Captain America or Robert Downey Jr. as Iron Man raises questions about the future of the franchise. But Bock notes that these were once B-list comic book characters that Marvel Studios transformed into A-list move stars. Bock also thinks the studio can keep the magic going, so long as it keeps casting extraordinarily well. "They really find actors who fit into these spandex like a glove, and it's hard to imagine them as anyone else," he said. Disney's pending 20th Century Fox buyout would also bring the X-Men and Fantastic Four franchises into the fold.

WATCH: Disney dominates opening weekend box office in U.S.