[Just for a little clarity: It wasn't my intention to say that the Westminster Standards are positively opposed to the idea of republication, rather, my argument is that the Westminster Standards don't positively teach republication.]

Dr. R. Scott Clark wrote on his blog in the first of a 3-part series on the republication of the covenant of works at Sinai:

Finally, it has been argued by some (e.g., some of my friends on the Puritanboard) that the doctrine of re-publication is “unconfessional.” To this I appeal to the logic implied by the grammar of WCF 19.1 and 2. 19.1 which reasserts the doctrine of 7.2, that God “gave to Adam a Law, as a Covenant of Works, by which he bound him and all his posterity to personal, entire, exact, and perpetual obedience; promised life upon the fulfilling, and threatened death upon the breach of it: and endued him with power and ability to keep it.” 19.2 says, “This Law, after his fall…was delivered by God upon Mount Sinai, in ten commandments….” (Articles, 30–31). The phrase “covenant of works,” in 19.1, is appositive to the noun “Law.” Thus the “Law” is reckoned here as a covenant of works. Thus when, 19.2 establishes “This law” as the subject of the verb to be, “was delivered,” the antecedent of “this Law” can be none other than the “Law” defined as a covenant of works in 19.1.

Here are some of the reasons why I believe this interpretation of the Confession is erroneous:

1. If the doctrine of a “republication” of the covenant of works at Sinai was taught in the Confession, the divines would have put it in Chapter 7 (“Of God’s Covenant with Man”), not in Chapter 19 (“Of the Law of God”). In Chapter 7 there is no suggestion of a republication of the covenant of works. Rather, it says that the Mosaic and the New Covenants are “not therefore two covenants of grace, differing in substance, but one and the same,” only under two different administrations (7.6). Thus the Confession explicitly says the opposite of the republication doctrine; the Mosaic Covenant is an administration of the covenant of grace, not the covenant of works.
2. WCF 19.2 says “this law,” not “this law, as a covenant of works.” The law is distinct from the covenant of works and 19.2 concerns the law, not the covenant of works. Nowhere in the Confession is the law equated with the covenant of works as though the law were itself a covenant of works. The “first covenant” (7.2) established a certain relationship between Adam and the law, as a covenant of works, but even the covenant of works was a “voluntary condescension on God’s part” (7.1). If the Confession intended to convey the idea of “republication” in 19.2, it would have read, “This law as a covenant of works, after his fall, continued to be a perfect rule of righteousness, etc.” But the Confession does not read this way. Thus the law per se continued to be a perfect rule of righteousness, not the law as a covenant of works.
3. In the interpretation of the Confession quoted above Dr. Clark states that, in reference to WCF 19.2, “the antecedent of ‘this Law’ can be none other than the ‘Law’ defined as a covenant of works in 19.1.” But why does this logic not equally apply to 19.3 which also speaks of “this law”? (19.3 reads: “Beside this law, commonly called moral, God was pleased to give to the people of Israel,” etc.) Why would the antecedent of “this law” in 19.2 be different from the antecedent of “this law” in 19.3? Clearly the “this law” of 19.2 and 19.3 must refer to the same antecedent or an inconsistency is introduced in the Confession. Thus, 19.2 and 19.3 both refer to the “law” given to Adam (19.1), but no longer as a covenant of works.
4. The Confession states that “true believers be not under the law, as a covenant of works, to be thereby justified, or condemned” and that the “promises of it [i.e., the law], in like manner, show them God’s approbation of obedience, and what blessings they may expect upon the performance thereof: although not as due to them by the law as a covenant of works. So as, a man’s doing good, and refraining from evil, because the law encourageth to the one, and deterreth from the other, is no evidence of his being under the law; and, not under grace” (19.6). The Scripture proofs associated with this section of the Confession (e.g., Ex. 19:5-6; Deut. 5:33; Lev. 18:5; Lev. 26:1-3; Ps. 19:11; Ps. 37:11) demonstrate that 19.6 describes the position of both OT and NT believers who are in the covenant of grace, not the covenant of works. Believers are promised rain, bread to the full, fruit on their trees, and safety in the land as blessings for their obedience “although not as due to them by the law as a covenant of works.” In this sense, according to the Confession, even OT believers were under grace and not law. These proof texts speaking of temporal blessings and afflictions are, according to the Confession, not to be interpreted to mean that OT or NT believers are under the law as a covenant of works. Those who adhere to the “republication” doctrine use these very same Scriptures in an attempt to prove the contrary of the Confession’s teaching.
5. The “this law” of 19.2 and 19.3 (which is synonymous with the “law” of 19.1, given to Adam, although no longer as a covenant of works as with Adam) is the “moral” law which “doth forever bind all, as well justified persons as others, to the obedience thereof” (WCF 19.5). While believers are “under the law” in the sense that all believers are required to obey it, true believers are “not under the law, as a covenant of works” (WCF 19.6, 2 times). OT believers are “true believers” and, if the covenant of works were republished at Sinai, then this would place true believers under the law as a covenant of works. This is contrary to the Confession. Further, if Dr. Clark’s interpretation is consistently applied, then the “moral” law is to be equated with the law as a covenant of works (since “this law” of 19.2 and 19.3 must have the same antecedent). This would place even NT believers under the covenant of works, which is also contrary to the Confession.

Any appeal to WCF 19 in the support of the republication doctrine seems to me illegitimate and contrary to the intent of the Westminster divines.