The election of 1980 was the most contentious since 1968. While he led by nearly 30 points early in the race, Carter's numbers started to go down and - for a time - it seemed like Reagan might win.
So, what if for whatever reason (a less successful Operation Eagle Claw would be a good PoD), Reagan beat Carter? Analysts say that the 80s marked a public disillusionment with decades-long New Deal policies, so I can easily see Reagan bringing in the Republican Revolution of 1984 four years early.
Would his first term go as poorly as Carter's second? Would his hawkish foreign policy bring an early end to the Cold War, or WWIII? One consequence I can see is that John Anderson's National Unity Party (if it still gets formed) would probably be consist more of moderate Republicans dissatisfied with Reagan than liberal Democrats fed up with Carter.