Sorry Nate, I could have given you some context. Up until now, Polybius had just been narrating events. But in Book VI, he digresses to discuss the Roman constitution. ἐπίστασιν is a noun here and has nothing to do with vetoing. εἰς ἐπίστασιν καὶ δοκιμασίαν is perhaps the clearest part of the sentence and crudely translates to "stopping and examining".
Here is one translation for the gist: http://penelope.uchicago.edu/Thayer/E/R ... us/6*.html
But I have my doubts about that translation because I find think the idea of testing what you are about to say rather loopy and I have a French one that is somewhat different albeit slightly evasive, "les développements qui vont suivre sur la constitution", ie "the developments that will follow on/from the constitution"--"développements" perhaps being richer in French than English.
Either the end of the sentence has to do with what he is going to say/what he believes, as that English translation reads, or it somehow has to do with effects of the constitution as the French one does.
But the more I look at it, I actually find the Greek clearer than the French or the English. I just think he is saying, "Now is the best time to talk about the constitution", but I can't quite tease that out of the Greek.
Actually, I just checked the master, ie Walbank, and he glosses εἰς ἐπίστασιν καὶ δοκιμασίαν as "for directing attention to what I am about to say, and putting it to the test". So, we should probably work with that. But I still can't unpack the Greek, especially at the end. But it is getting late.
(If you get a chance someday, check out Walbank's three volumes, one of the true high points of 20th century scholarship.)