One could argue that entities and revolts such as the
Comune of Rome could point that urban middle-classes had chances to weight in the political structuration, but again the differenciation between middle and upper classes isn't obvious politically, and still had to rely on a greater support.I'd rather have
@Carp 's opinion on this, tough.
In Rome, at least, there was a fairly clear (although not absolute) differentiation between the
nobiles or "true" aristocracy and what Chris Wickham terms the "medium elite," which is as good a phrase as any. These were the local elites of Rome's
regiones and the key players in the Roman Senate. Although many of them were tradesmen of one kind or another (butchers, masons, smiths, etc.) what tended to define the "medium elite" was a diversification into agriculture via leasing ecclesiastical and monastic land, and the accumulation of various administrative offices in the Church (notaries, judges, various kinds of stewards). A member of this class had no allodial estates or "fiefs" properly so called, but they clearly could live comfortably on a diverse income derived from some mixture of trades, rents, agricultural production, and administrative fees. They were an armed class as well, possessing weapons and armor and composing the city's
militia, and owned fortified or tower houses of their own.
This class of people existed in other cities (although the size of the Roman bureaucracy meant that they were rather more numerous than elsewhere), but what seems to have been different in Rome is that instead of allying
up, they allied
down. Whereas in other communes such upper-middle class citizens tended to aspire to the aristocracy and to some degree merged with them, excluding the orders beneath, the Roman upper-middle class considered the aristocracy to be its enemy and tended to look for allies from their lessers among the "middle class" properly so called. In Rome, the economy was dominated by the Church - note that both of the means of advancement to the "medium elite" I mentioned came through the Church, either by leasing its land or holding offices in its administration. But while the Church administration was
staffed by this upper-middle class, it was
ruled by a curial-aristocratic club, families of the
nobiles who occupied both lay and ecclesiastical posts at the top of the hierarchy. The totality of the Church's economic dominion and the iron grip which the aristocracy exerted on the Church and its high offices created a situation where the medium elite's ambitions were frustrated, causing them to see the aristocracy as an enemy to be overthrown rather than a social club to rise into.
Accordingly, the initial rebellion and establishment of the commune was directed against the curial-aristocratic alliance that held the commanding heights of political and economic power. They usurped the Pope's
regalia in the city and claimed it for the Senate, abolished the office of the urban prefect (generally an aristocrat appointed as the city's administrator by the Pope), and tore down the towers and palaces of the aristocrats (termed the
nobiles and
potentes urbes) and high-ranking clergymen. They were not opposed to the church as an institution - they derived much of their benefits and livelihoods from it - but wished to control it themselves and remove the parasitic aristocratic elite at the top. One could look at the Commune, particularly in its original intent, as an effort by what you might call the "upper-middle class" to advance from merely
staffing the Church bureaucracy to actually
controlling that administration and its considerable revenues directly.
As mentioned, this leading class of the Commune looked for support and assistance from below - not to the
truly lower classes like common laborers, but to the fringes of the medium elite and lesser artisans and tradesmen who represented the main body of the "middle class" as you might call it. But these men were not the motive force of the revolt, and probably could not have been. In the first place, the medium elite was already an armed (and armored) military body. Additionally, their grievances against the curial-aristocratic order were more acute, because they already moved within the Church administration as its estate managers, renters, and bureaucrats; they understood how the aristocratic elite controlled the wealth of the Church, could personally resent their dominance, and could credibly contemplate overthrowing them. Lower artisans and tradesmen with no presence in the Church structure either as renters or administrators were not in the same position to either be aggrieved by or seize the power of the bishops and aristocrats. It may be this larger body of the "middle class" - or even lower orders than this - which was subsequently motivated by political Arnoldism some years later; Arnold was said to be influential in the public assemblies, and to have stirred up a "rustic mob without aristocrats or elites."
So here, at least, you
sort of have an example of a "patricianate" going "plebian," but with qualifiers - the class which led the Commune was not a
true patricianate but an aspiring upper-middle class, and the "plebians" they allied with were not laborers, peasants, or indigents but the lower orders of the middle class. This example may be difficult to repeat, however, because it appears to have been unique to Rome and its curious political and economic situation. Rome had a fairly numerous, well-educated, and well-armed administrative upper-middle class which found itself locked out of power by an impenetrable aristocratic oligarchy, and this condition can't be found everywhere.