The Nocturnal Animals star appeared with the comedian in a skit inspired by Charles Dickens' A Christmas Carol on Monday night's episode of Colbert's Late Show.
In the segment, Colbert plays an Ebenezer Scrooge-inspired miser awaking on the eve of Election Day, much to his delight.
"Last night I dreamt I was visited by three ghosts! There they are," he says as the camera turns to framed pictures of Republican politicians Mitch McConnell, Chuck Grassley, and Orrin Hatch.
Colbert's character then steps outside and asks a "boy" – a confused Gyllenhaal, who reminds Colbert he's 37 years old – what day it is.
"It's Monday. The day before the midterms," Gyllenhaal says.
Colbert rejoices in the fact that "there's still time to vote", but the conversation rapidly switches to his nightgown and the lack of coverage it provides.
"You know I can see up your nightgown, right?" Gyllenhaal asks.
US midterm elections: voters head to the polls
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He adds: "Seriously, dude, you've got to put on some briefs. I can see all the election returns and they're swinging left."
Colbert, however, remains oblivious to Gyllenhaal's complaints and instead rejoices in the fact that Election Day is nigh.
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The host is set to go live on Tuesday to cover the midterms, as are his late-night colleagues Jimmy Kimmel, Seth Meyers, and Trevor Noah.
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