Like any modal argument for theism that uses S5, the modal cosmological argument (MCA) is sometimes alleged to commit the informal fallacy of begging the question. First, here's a refresher of the argument:
1. Every contingent entity possibly has an external cause. (Premise)
2. If the sum total of contingent entities C has an external cause, that cause is a necessary entity. (Premise)
3. C is a contingent entity. (Premise)
4. Hence, a necessary entity possibly causes C. (From 1 - 3)
5. Therefore, a necessary entity exists. (From 4 and S5)
Unless the skeptic is willing to question the truthfulness of S5, or else either of (2) or (3), which in any case would be highly dubious, he must focus his attention on (1). Does (1) beg the question?
It's not entirely clear. (1), on its own, does not entail (5), even in conjunction with S5. (2) and (3) have to be added, and only (2) is plausibly analytical. (3) seems almost indubitable, but that doesn't (or shouldn't) have any effect on one's acceptance/denial of (1). In other words, it is coherent to simultaneously affirm (3) and reject (1).
What makes the circularity objection even more suspect is that the negation of (1) does not result in the negation of (5). Even if it is not possible for every contingent entity to have an external cause, a necessary entity may very well exist. This is peculiar if the MCA's proponent is already assuming the truth of (5) by postulating (1).
Finally, even assuming that (1) entails (5) - which it doesn't - that doesn't undermine the argument. There are often cases in which one statement will entail another, despite their possessing two distinct levels of perspicuity. Take, for example, the proposition, "Socrates is a man." This statement is much more obvious than, "Socrates is a homo bipedal primate." Yet, the former entails the latter. What this illustrates is that some statements can supplement by their clarity even those statements they entail. Why can't this be the case with (1) and (5)?