Whenever a man has cast a longing eye on offices, a rottenness begins in his conduct.

Jefferson is warning that the pursuit of public office for its own sake—status, power, patronage—tends to corrupt character. The “longing eye” suggests ambition that becomes a fixation; once a person begins to angle for preferment, their behavior can subtly shift toward calculation, flattery, factional maneuvering, or compromise of principle. The image of “rottenness” implies moral decay that may start invisibly but spreads, affecting judgment and integrity. Read in a republican key, the line reflects an ideal of civic virtue: offices should be accepted as burdens of service, not coveted as prizes. It also functions as a critique of careerism in government and a caution to voters about candidates driven by personal advancement.