Actually iirc Regan was behind in the polls up to election day when he cleaned Carter’s clock.
But to address why the Dems are so confident- I would describe it as an overdose of the kool aid. Remember these are people who fell hook, line, and sinker for a campaign that had nothing more than Hope and Change to sell as a platform. Now they are peddling the same nonsense coupled to a dismal record and consider it a brilliant campaign. I think it is a matter of delusion.
So basically, every poll in the few days before the election has showed the Democrat winning since the 1980 elections, regardless of who actually won. And so anyone saying Obama is winning among polls favoring Democrats by +10 are not being accurate in the slightest. True ? I sure hope so !