Nixon put up a big lead, actually... but Humphrey managed to close fast toward the end, as things in Vietnam started to improve. If the campaign had lasted another week, Humphrey would probably have won.
Not true. Humphrey got the last minute bounce from the October 31 bombing halt ordered by Johnson. Over the weekend, however, it became clear that South Vietnam would not participate in the sham peace talks engineered for political purposes, and the election was drifting back to Nicon. Another week, and Nixon would have won decisively.