Barry Stocket at New APPS Blog has an interesting piece noting how the decadence of the republics known to late 18th century Europeans discouraged them from considering the republic as a suitable form of political organization for those interested in implementing Enlightenment thought.
The idea of democracy was even more anachronistic looking before [the late 18th century revolutions, which in any case did not lead to full implementation of democracy and certainly not its normalisation, but did take steps in that direction. Earlier in the eighteenth century, even Rousseau did not think of democracy as the ideal. Even allowing that his advocacy of elective aristocracy is in accord with representative democracy, it does not look as if he expected republicanism to sweep through Europe. His text on a constitution for Poland was for an aristocratic state on the verge of extinction as Prussia, Russia, and Austria arranged its complete partition between 1772 and 1795. It was not a model for European republics, nor was it any more democratic than the existing aristocratic commonwealth with a limited monarchy. Montesquieu, Smith and Hume looked upon republicanism as a form of government appropriate to liberty, but not as necessarily superior to monarchy, and maybe less desirable than monarchy in the circumstances of most modern states.
What was wrong with the republican model for eighteenth century thinkers about liberty, if they themselves have sometimes been taken up as republican, or at least partly republican, thinkers? The answer can be summarised with reference to a city state where Rousseau himself spent time as a secretary to the French ambassador, Venice. Those looking for an example of a modern republic in the eighteenth century were likely to look at Venice. Another possible example was the Dutch Republic, but at least for Montesquieu (who I will take it was not deviating far from any prevailing judgement) it was a more a confederation of city republics which struggled to achieve unity in times of danger. By the eighteenth century the glory of the Golden Age, of Rembrandt and Spinoza, of recent independence after a long war from Spain, of a model of financial and commercial progress, was in the past, and no one thought of the Dutch Republic as a major European power.
Furthermore its relative youth, going back no further than the 1560s, mattered to eighteenth century thinkers who though that successful model states are states that maintain themselves over centuries, preferably with a largely unchanged constitution. That Athens had only a couple of centuries maybe as an independent and democratic republic was important compared with the much longer life of Sparta’s oligarchic republic. That Roman republicanism gave way to a thinly disguised version of monarchy in the Emperor system after five centuries mattered, as did the apparent weakening of republicanism and democratic life after the defeat of Carthage.
Two centuries of republican life in the United Provinces was small compared with about one thousand years of the Venetian Republic, which like the Dutch Republic was past its greatest period of influence, but could be taken as more of a model with an apparently little changed constitution over a long life by the standard of any European state. By the tine of Enlightenment political writing Venice was a museum of a glorious past as a dominant commercial and naval power, with an eastern Mediterranean empire. Montesquieu comments unfavourably on a constitution which he thought allowed the aristocracy to act as government executive, legislator, and judiciary, with the special powers of secret committees to defend the state undermining liberty.
His criticism is more than justified by the greatest Italian witness of the time, Giambattista Vico (whose thought anticipates much in Rousseau and Montesquieu). Vico thought of Venice as the model of aristocratic republic in which the aristocracy regards itself as more than human and the common people as less than human. It precedes the situation in which democracy encourages the spread of an idea of a common humanity, an idea that Vico thought could only maintain itself through democracy giving way to a human monarchy, legislating and judging with regard to the welfare of all.