Ok. So Reagan beats Ford in '76, either through a slightly better primary performance or at the convention.
Reagan versus Carter is close, but Reagan cuts deep in Carter's Southern base and retains enough other support to win.
With no Carter there's no Mondale (he tried once, before being VP, and decided running for President wasn't for him), so the 1980 field is wide-open.
For whatever reason John Glenn wins. Doesn't matter the details, for a rough sketch.
Up against Reagan Glenn has basically conceded much of the South, but unlike Mondale in '84 and Carter in '80 Glenn has a strong appeal to the same blue collar Democrats that Reagan needs. Glenn takes either Jerry Brown, Governor of California, or some Southerner (preferably from Florida or Texas).
It's a close race but sweeping the Northern states and a surprise victory in California or Texas gives him the Presidency.
Since Glenn was your bog standard moderate Democrat I can't see that much changing.
You do know that Glenn ran for President in '83/'84, right? He was polling second to Mondale early on.
The Right Stuff was released early in the campaign. Perhaps he could have capitalized on it more?
Personally, I think he was just too nice to be President.
We talked about John Glenn in '84
here. I suppose he could have won the primaries, but he probably would have lost to Reagan.