Possibility?
In 1983, Reagan refuses to re-nominate Paul Volcker for Chairman of the Federal Reserve. Maybe for a more accurate de-regulator, possibly Greenspan or someone else. The economic recovery does not fully happen, and without Volcker's tight fiscal policy plus high deficits, inflation begins creeping back. At the same time, throughout 1983, unemployment is at a high of over 9.5%.
In the Democratic primaries, Mondale is able to quickly beat out possible Democratic challengers, and chooses national hero John Glenn for Vice-President, thus giving his campaign an extra boost. Despite low Reagan approval ratings, Mondale is in no way guaranteed the Presidency thanks to Reagan's charisma and evidence of a slowly improving economy, as seen by unemployment now being in the sevens. However, Mondale, despite a lack of charisma, is able to hammer Reagan in the debates over high deficits, the slow recovery, and cuts to things like social security. On election day, Mondale wins a narrow victory.
Not sure about the nuances, but this is an idea. I'm no award winning economist, so forgive any possible mis-statements.