In 1980 Carter was leading and lost due to the final debate. Keep in mind there isn't much of a sample to choose from. The only incumbents to lose in the postwar polling era have been Ford, Carter, and Bush I. Ford and Bush I were trailing mightily well before the debates, and so they didn't have a lead to lose. Of the winning incumbents, most didn't debate (Truman, Eisenhower, Johnson, Nixon '72) and of the ones that did, they were considered to have won their debates (Reagan, Clinton).
Carter was the only one with a lead who lost his debates and he also then lost the election.
i think youre right but carter was well under 50%, Bush has been at 50% or better for nearly one month.
Problem is Kerry came off much better than people expected. he wasnt as bad as he has been.
Carter didn't loose because he lost the debates. He lost because he was the worst President in 30 years with double digit interest rates, soaring unemployment, High gas prices with lines, loosing control of Iran and waiting 11 months to even attempt to rescue the hostages held in Iran after the Shah fell and then botching it. etc etc.