Firstly, Portmore argues that we need our moral theory to assess the permissibility of act sequences, not just individual acts. He mentions a case in which a patient needs either one of two drugs, but the combination would be lethal. It then seems that the doctor is permitted to give either drug -- each act is permissible considered individually -- but it's impermissible to perform both acts together. Contra Portmore, however, I do not think that this sort of case provides any support for the conclusion that an ethics of individual acts is insufficient.
Instead of shifting our attention to assessing act-sequences, we may simply need to build more information into the token acts that we wish to assess. For example, having already given drug A, the token act of giving drug B is now straightforwardly impermissible. But if we hadn't given the patient drug A, then we would now be faced with the prospect of a different token act of the type "Giving the patient drug B", and this particular token action would be permissible (perhaps even obligatory). So you see there is no need to shift to act-sequences. Portmore rightly recognizes that it's insufficient to assess the permissibility of an unspecific individual act type, but fails to appreciate that this problem is easily solved by instead assessing the permissibility of completely specified individual act tokens.
Not only is the shift to deontic assessment of act-sequences thus lacking in motivation, we may also adduce positive reasons for thinking it incoherent -- as I argue in my old post, 'Only Action is Practical'.
On now to the Actualism-Possibilism dispute. Portmore begins with the standard objection that Actualists posit incompatible obligations. I respond to that charge here. In short, once we reject the misguided application of deontic terms to act-sequences, there is no incompatibility at all. At each moment in time, there is a single thing that the agent ought to do, namely whatever would make things turn out best.
Portmore further objects: "actualism holds both that agents ought to pander to their own vices and that they can limit what's required of them by doing so." This is misleading. Obviously if an agent has the option of ridding himself of a terrible vice (at no comparable cost) then Actualism implies that this is just what he should do. The view just is that an agent should, at each time, perform the act of those available that would actually be for the best. This may well include character development. So Portmore misrepresents Actualism when he later claims that it vindicates Tony's "reasoning on the basis of assumptions [about his future viciousness] that are only true because he has acquiesced to [the] idea that he'll behave badly in the future." If Tony's future bad behaviour is contingent on his current acquiescence, then Actualism will straightforwardly imply that what he should do right now is stop acquiescing!
So, fighting our vices (so far as we are able at any given moment) is great. What Actualism instead rules out is wishful thinking, or imprudently ignoring the likelihood of our well-meaning plans being subsequently (and perhaps disastrously) derailed by our moral flaws -- flaws that we can't currently undermine. When deciding what to do at time t1, we must take into account what we will likely do in the future. We should, of course, seek to influence our future selves in positive directions insofar as we are currently able. But insofar as our future decisions are not under our current control, it would be myopic to just assume in our current deliberations - as possibilists advise - that our future selves will do everything they ought. That way lies disaster.
[Aside: it's worth noting that any moral theory implies that an agent can, by acting wrongly now, limit what they are required to do in future. This is a straightforward consequence of "ought implies can".]
The central flaw in possibilism is that it conflates what is in an agent's control at some time or other with what is under the agent's control at the current moment of decision. For example, Portmore claims that it is "absurd" of actualists to claim that Angry Tony (who will later choose to beat his son to a pulp if he doesn't now let off steam by slapping Valerie) "ought to give into his temptation to hit Valerie even though he could refrain from hitting anyone." This is misleading. "Refraining from hitting anyone" is not among the options that Tony faces at the present moment. His current choice is to slap Valerie or not, and if he does not, he will later face a new choice between beating his son or no-one. Actualists agree that, in assessing this later choice, he ought to hit no-one. (It would indeed be "absurd" to deny this.) But to assess the present choice, we need to hold fixed what is not within the scope of his present control. And if he will actually later choose to beat his son (or if this is overwhelmingly likely) then Tony's present choice is between slapping Valerie or else bringing it about that he will (overwhelmingly likely) beat his son. Since all admit the stipulation that the former outcome is preferable, this is what he should choose. It is not "absurd" -- quite the opposite.
Next, Portmore considers the Correlativity Thesis: S1 has a right against S2 that S2 φ iff S2 has an obligation to S1 to φ.
[I]f CT is true, then Valerie has a right against Tony that he refrain from hitting her only if Tony has an obligation to Valerie to refrain from hitting her. But if actualism is true, then Tony has no such obligation... This is quite implausible, not only in its own right, but also because it makes it unclear why Tony owes restitution to Valerie, which he surely does.
This is no objection to Actualism. Just tweak the case so that Tony won't have any later choice about whether or not to beat his son -- suppose he'll be under a compulsion if he does not now slap Valerie. So now even Possibilists will agree that Tony ought to slap Valerie. Yet he owes her restitution all the same. Obviously something is wrong with (Portmore's interpretation of) CT. Either CT is false, or else Tony can have a (pro tanto) obligation to Valerie to refrain from hitting her even though hitting her is what he ought (all things considered) to do. Either way, the objection to Actualism cannot be maintained.
Finally, Portmore discusses the case "Ten Years Later", whereby an agent buys a car and then ten years later decides to drive while sleepy, and ends up in a fatal accident:
Actualism supposes, contrary to commonsense, that all of those [earlier] decisions were wrong merely because she wouldn't have made the bad decision to continue to drive had she not made the other seemingly good decisions. This just ignores the fact that she could have made all those good decisions without having made the bad decision to continue to drive.
This is an unhelpful case, since one might naturally complain that the agent had no reason to expect a bad outcome from any of her prior decisions. So an Actualist who is also an Expectabilist (privileging the "evidence-relative" spot on the epistemic dimension) is not, in fact, committed to thinking those earlier decisions were wrong.
In addressing the arguments for Actualism, Portmore dismisses the significance of benevolent spectators:
[F]rom the fact that we ought to advise Professor Procrastinate to decline the invitation, it doesn't follow that he ought to decline. These two can come apart, for whereas what we ought to do depends on what's the best that we can do, what he ought to do depends on what's the best that he can do.
It may be unfortunate that Actualists have spoken of the act of advice, since a spectator may have reason to give insincere advice if that would lead to better results than speaking the truth. But that isn't what's happening here, as we can see by skipping straight to the benevolent spectator's preferences. If God (or anyone familiar with Prof P's flaws) sees Prof. Procrastinate accept the invitation to review, she will feel like banging her head against the wall in exasperation with PP's lack of foresight. It is clear, in other words, that by agreeing to review the book, PP is making a bad (regrettable, irrational, whatever) decision.
Portmore goes on to claim that Procrastinate, unlike us spectators, can "control what he'll do if he accepts." That is to say, at a later point in time that will be under Procrastinate's control. But it isn't presently, and the present (as I keep insisting) is when he has to make this decision.
We've seen that Actualism makes claims that can sound absurd, insofar as they are easily mistaken for different claims that Actualists do not actually make. No such misrepresentation is required to bring out the problems with Possibilism. For example, in my Saint-Or-Sinner case, it implies that the agent should accept a deal that will (given their moral imperfection) be almost certain to lead to the torture and deaths of millions of innocents. This seems a bad choice to make in the circumstances, to put it mildly.