This has been a question I've thought about recently, what would have happened if instead of accepting the American Independent nomination Wallace had joined the Democratic primary in 1968?
So Wallace decides that instead of taking a chance with a pretty much unknown political party, he takes another swing at the Democratic primary. It's not really implausible, since Wallace was in the Democratic primary in 1964 against Johnson, who now has a hell of less support than in 1964. In TTL, Wallace sees a better opportunity for round 2 against Johnson and runs against him as a Democrat.
How would this effect the primary? Would it bring Robert Kennedy in the race earlier? Would that knock out Johnson before the New Hampshire primary? And finally how does this effect the general election?