Romney was criticised by some in the media for appearing too aggressive and frequently interrupting Barack Obama in a town hall-style forum at Hofstra University.
Hamill discussed the fall-out of the debate on Current TV's The Young Turks on Wednesday (October 17), revealing that he found Romney's performance disturbing.
"I love the idea of an open debate like in France... but this sort of repeated... it's almost like harassment," the star claimed. "The president's trying to get his answer together, and [Romney interrupts] because it's not coming fast enough for the CEO."
Hamill continued: "[Romney interrupts] again and again... It's arguably disrespectful. Patronising beyond all belief... It's bullying, to me, of the highest degree!"
The actor accused Romney of lying "shamelessly" last week in a Twitter post, in which he also referred to the candidate as a "snake oil salesman".
"Unlike the GOP, President Obama cares about more than the wealthy, he cares about us all. Obama deserves a 2nd term," Hamill tweeted.
Samuel L Jackson, Jay-Z, Sarah Jessica Parker and George Clooney are among the other celebrities who have endorsed Obama.
The US presidential election is held on November 6.