Romney was in the midst of his opening debate remarks, when the audience began cheering his vow to "restore America's promise".
The former Massachusetts governor acknowledged the cheers by declaring: "As [Seinfeld character] George Costanza would say, 'When they're applauding, stop'."
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The line Romney quoted from Seinfeld was not actually spoken by Alexander, but his co-star Jerry Seinfeld.
In the 1998 episode 'The Burning', Jerry tells George: "Showmanship, George. When you hit that high note, you say goodnight and walk off."
Alexander - who played George Costanza on Seinfeld - responded to Romney by posting a series of messages on his official Twitter.
The actor retweeted several 'Costanza 4 Pres' posts from fans and also mocked Romney's switch from some liberal political positions to more conservative stances in recent years.
"Thrilled that Gov. Romney enjoys my old character. I enjoyed the character he used 2 b 2. If he'd embrace that again, he'd b a great candidate," Alexander wrote.
Alexander has continued to post 'Constanza 4 Pres' slogans since the CNN debate.
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Watch Mitt Romney quote Seinfeld below:
