Baker and Kemp would win in a fairly close race. The country was still pretty conservative in 1984. This is providing that Reagan enthusiastically supported the ticket.
There are many reasons why Reagan might not have run. Failing health, the onset of Alzheimer's, lingering effects from the assassination attempt. Nancy didn't want him to run again. If she had been more insistent about it, he might not have. It's hard to imagine why Bush wouldn't run, though, and how he would fail to get the nomination if he did.
The two tickets wouldn't offer much to liberals. I wonder if they would mount a third-party effort, perhaps with Jesse Jackson on the ticket?
Biden offers something to the liberals. He is one of the most liberal Democrats.