Here’s a line allegedly from Montesquieu’s Spirit of the Laws that I’ve only ever actually seen quoted in other works:
As Rome, Sparta, and Carthage have lost their liberty and perished, so the constitution of England will in time lose its liberty, will perish: it will perish, whenever the legislative power shall become more corrupt than the executive.
I’m not sure this means American liberty is doomed, but I think it would be hard to deny that the corrupting influence of special interest politics weighs heavier on the Hill than it does on the White House.
Meanwhile, I do think you can see an inkling of what Montesquieu is talking about in the fact that there’s a persistent impulse in the contemporary United States to say that if something is really important, we need to basically cut congress out of the loop. This probably happened first with the steady decline of congress’ war powers. But you also saw it in the way that the Treasury/Fed response to the financial crisis was shaped by an overwhelming desire to avoid the need to go back to congress, by the way that proposals for improving the operations of MedPAC all involve trying to circumvent congress, etc. Tellingly, the judgment that congress can’t handle these issues is a judgment largely shared by congress. In the England of Montesquieu’s day, of course, the “executive” was understood to mean the unelected King, so a shift in the balance of power from legislature to executive constituted the death of liberty. Here in the US, obviously, it’s a different situation.