It's an interesting question because 1968-1972 saw the labor union demographic begin to swing independently of union directives. IOTL a majority were still following the union line, but by 1972 various factors (I'm going to blame the generation gap, personally) saw a majority swing against the Democrats. This was the birth of what would become Reagan Democrats.
So can he make them swing early? They were on the cusp before. But if I'm right and the breech is primarily about the generation gap (with McGovern seen as kowtowing to younger urban liberals) then they'll probably stick with Humphrey for now. So Reagan-as-disruptor just doesn't happen in 1968.
Looking to the South, I'm not sure having two very conservative candidates and one moderate could *ever* be said to play into the hands of either of the two conservatives. Looking at Nixon's OTL totals as a baseline, Reagan would have a LOT of work to do to take over any of the states Wallace won, with Arkansas being maybe the only target. Texas is a big prize he might have a shot at that went narrowly for Humphrey IOTL. But I don't think even Reagan is willing to make the kind of deal necessary for Wallace to agree not to run. (If he did, he'd be toast for sure.)
How well does the Goldwater strategy play against Reagan four years later? I think that's the pattern everyone sees, and they either say, "these guys are still too crazy for us," or they say, "maybe we should've given it a try." I'm inclined to agree with PresidentZinn, that the Overton Window is going to keep people from experimenting with more cavalier approaches to the Cold War- I really think you need a defeat in Vietnam to prep the nation for that.
So I think a larger percentage of Middle America pulls for Humphrey out of fear with little upside for Reagan outside of perhaps a couple of additional southern states (one of which is probably Texas). There were SO MANY close states in 1968! By my count Humphrey lost eight states by 3% or less. A 1.6% swing his way from Nixon is all it takes to win a comfortable victory. At least that much seems rather likely. A 3-5% swing is well within the realm of possibility and gives him a blowout.
I then have to wonder what happens to the conservative movement with two terrible losses in a row. Does the money stop flowing to those quarters? Do we see refinements in the ideology? Does the burgeoning conservative coalition manage to stay together under the strain?
Reagan's still got a state to run, so he could make a comeback later. Assuming his base of support is there.