Clinton was asked Saturday at a news conference in Jerusalem about the presidential challenger's decision to not participate in the runoff.
She said that it his decision to make but said the runoff has already been legitimized.
When Afghan President Hamid Karzai accepted the runoff after the first election, she said, "that bestowed legitimacy from that moment forward."
State Department spokesman P.J. Crowley said the matter of when to hold a runoff was a decision for the Afghans, and they have already said it should be Nov. 7.
Asked about putting it off until spring, Crowley said, "It's not our decision," adding, "The sooner a new government emerges, the better."