Find the part of the sentence that contains an error: What renders the draft…

Concept: In a contrastive construction of the form 'is not X, but Y', the two elements X and Y introduced by 'not' and 'but' must be grammatically parallel -- both noun phrases, both clauses, or whatever single structure the first side sets up. Introducing a subordinating word on only one side (such as 'that') breaks that match.

Application: Here, X is the noun phrase 'its proposed realignment of fiscal powers per se'. For 'is not X, but Y' to stay parallel, Y must also read as a noun phrase. Part (2), however, is 'but that it presumes, without empirical substantiation,' -- the word 'that' turns this segment into a dependent clause rather than a matching noun phrase, breaking the parallel structure the construction requires.

Cross-check: The remaining parts hold their own grammatical ground:

• Part (1), '...is not its proposed realignment of fiscal powers per se,', uses 'its proposed realignment of fiscal powers per se' as a single noun phrase functioning as the complement after 'is not'.

• Part (3), 'a fiscal equivalence among states whose developmental baselines are', opens with the noun phrase 'a fiscal equivalence among states' and continues into the relative clause 'whose developmental baselines are', which completes in Part (4): 'whose' correctly links 'states' to 'developmental baselines', and the plural 'baselines' correctly takes the plural verb 'are'.

• Part (4), 'incommensurable by any normative metric', is a grammatically sound predicate-adjective phrase that completes the sentence.

Result: Part (2) is the only segment that disrupts the noun-phrase parallelism set up by 'is not ... but', which is why it is the part containing the error.