I think people forget just how bad Reagan was in that first debate: claiming that he "started going to church here in Washington", although the debate was held in Louisville, Kentucky, referring to military uniforms as "wardrobe," and -- worst of all -- admitting publicly to being "confused" throughout the debate.
This was October 7, 1984. The second debate was October 21, just two weeks before the election.
Now, that's probably not enough to cause anyone to lose to Walter Mondale, but my guess is that you'd see wholesale and widespread panic in the Reagan campaign. He's probably shed as much as 10 points over a two-day period -- Obama dropped 3-4% on the basis of his lackluster debate performance in 2012, for purposes of comparison -- maybe even enough to throw enough states into "Leans Mondale" + "Too Close To Call" to total 270 EV.
The press would declare it a whole new ballgame, anybody's race to win (which is always in the press's interest, regardless of the candidates). The fundamentals would still strongly favor Reagan, but the entire tenor of the campaign -- which has been "REAGAN IS INEVITABLE" for all of 1984 -- will change.
And there's just two weeks to go!