The new footage for the two-part, four-hour docudrama features Gleeson's impressive transformation into Trump, alongside Jeff Daniels as former FBI Director James Comey.
"The Comey Rule," which is based on Comey's bestseller "A Higher Loyalty," is described as an "immersive, behind-the-headlines account of the historically turbulent events surrounding the 2016 presidential election and its aftermath, which divided a nation."
"I know some people at the bureau have it in for me, but I'm the president now," Gleeson's Trump begins in the teaser.
"Why are there four guys with ties to Russia on one campaign?" Daniels' Comey asks.
Gleeson's near-perfect portrayal is then heard saying, "Nobody gets treated as unfairly as I do, I need loyalty."
"I have looked at this from every angle," Daniels' Comey later adds. "They're all terrible. We're steaming toward an iceberg."
The short clip ends with Gleeson's Trump and Daniels' Comey shaking hands.
According to Showtime, part one of the miniseries follows the Russia investigation, the Hillary Clinton email controversy and Trump's victory in the 2016 presidental election, while part two presents a "virtual day-by-day account" of the demise of Comey and Trump's relationship.
Although based on Comey's autobiography, Showtime says the series "is not a biopic of one man, but is instead the story of two powerful figures, Comey and Trump, whose strikingly different personalities, ethics and loyalties put them on a collision course."
The drama also stars Holly Hunter as former Acting Attorney General Sally Yates, Jennifer Ehle as James Comey's wife, Patrice, Kinsley Ben-Adir as President Obama, Michael Kelly as former FBI Director Andrew McCabe, Peter Coyote as Robert Mueller, William Sandler as former National Security Advisor Michael Flynn, Michael Kelly, Joe Lo Truglio as former Attorney General Jeff Sessions, Jonathan Banks as former National Intelligence Director James Clapper.
Watch the full trailer, above.
"The Comey Rule," a two-night series event, premieres Sunday, September 27 on Showtime.