I think the election would in that case be somewhat similar to where we may be going in 2012. Assume something of a recovery, but a moderate one. GDP growth 3-4% instead of 7-9%: in that case you have a poor economy, not fixed by the current guy but also the responsibility of the previous guy of the other party. You have a candidate who campaigned on optimism and ideological generalities, considered too far from the center by the opposition during the campaign but nevertheless won vast numbers of independents.
The most significant difference would be that Reagan did not spend the first two years of his administration focused on an unpopular domestic measure without a clear tie to relieving the economic/unemployment crisis. The opposition base would instead be fired up by the less widespread rallying cry of the nuclear freeze movement. The primary argument from the mainstream though would simply be "Reaganomics didn't work."
I think in that case, it would depend on the candidate. Dismissing him as "the Republican Carter" isn't all that simple, as Democrats are still saddled with the Democrat Carter. The incumbent Reagan would probably be favored in a close race, though the right candidate could defeat him with the right message.
Mondale would not be able to do it. He was too far to the left. The candidate doesn't just need to retake OTL Reagan Democrats, but also be centrist enough to appeal to Rockefeller/Anderson Republicans, who could possibly be enticed in this scenario. The best chance, therefore, would be with Gary Hart.