Stumbled upon this in my list of quotes... true then, true now.

"A nation can survive its fools, and even the ambitious. But it cannot survive treason from within. An enemy at the gates is less formidable, for he is known and carries his banners openly.

But the traitor moves among those within the gate freely, his sly whispers rustling through all the galleys, heard in the very hall of government itself.

For the traitor appears not a traitor...He speaks in the accents familiar to his victims, and wears their face and their garment, and he appeals to the baseness that lies deep in the hearts of all men.

He rots the soul of a nation...he works secretly and unknown in the night to undermine the pillars of a city...he infects the body politic so that it can no longer resist. A murderer is less to be feared."
-- Taylor Caldwell, playwright, often incorrectly attributed to Cicero

Hat tip to mahatmakanejeeves, in post 10 below, who researched the quote and pointed out the correct attribution.