There is the following sentence in the conversation between Florentyna Rosnovski, the heroine of Jeffrey Archer’s novel, The Prodigal Daughter, who was first elected as the Congressman of Illinois and her husband, Richard Kane, Chairman of a New York bank.
She captured the Ninth District of Illinois with a plurality of over 27,000 votes. Richard was the first to congratulate her.
"I’m proud of you, my darling." He smiled mischievously. "Mind you, I’m sure Mark Twain would have been as well."
"Why Mark Twain?" asked Florentina, puzzled.
"Because it was he who said: 'Suppose you were an idiot and suppose you were a member of Congress. But I repeat myself'." – The Prodigal Daughter, Page 302.
I can’t get the idea of the last phrase, “But I repeat myself,” following the preceding subjunctives – Suppose you are idiot or a member of Congress. Is Richard simply saying “Whoever you are, I won’t change.”?
What is the meaning of it as the punch line of a famous quote from Mark Twain?