April 1999, Gallup Poll Bush 54 Gore 41.
But I discount polls like this and tend to focus on the "Fundamentals"
Clinton 60% approval ratings,
GOP lost seats in 1998,
Economic Fundamentals Strong: Inflation, Unemployment, Fiscal Budget.
Gore should have won by 5-8 points like Daddy Bush in 1988.
One of the worst winning campaigns was also Nixon in 1968. He sat on a 15 point lead for much of the summer and almost blew it.
Id rate the best losing campaigns in no order as:
Romney 2012 (really made no public mistakes, got more votes than the GOP in 2008 and won 2 ore states).
Ford 1976 (his debate gaffe may have cost him, other than that ran a strong campaign considering where the GOP was in the 1974-76 time frame).
Humphrey 1968 (Came from 15 points down in an absolutely awful year for Dems).
Nixon 1960 (He could have called out JFK on the missile gap lie, but would have had to reveal classified info to do so).
Romney made no public mistakes? What about 47%? He was also kind of a gaffe machine with his constant out of touch remarks.