First of all, to disabuse the idea that there was a middle class before the French Revolution, the feudal system was existant in France. As such, there was an aristocracy who owned the land with a peasantry who worked on this land. This was the First Estate. The Second Estate was composed on the clergy; only they and the First Estate had any political power. The Third Estate which consisted of the peasantry along with the merchants and lawyers wanted recognition and voting rights, especially the merchants and lawyers; and for this reason they urged on the revolutionaries.
Charles Dickens, a man of democratic principles who was sympathetic to the plight of the poor since his own father had gone to debtor's prison and he had to work as a child, was sympathetic to the oppressed everywhere, certainly. Dickens's social ideas in A Tale of Two Cities are very basic: The French Revolution was inevitable because the aristocracy had exploited and plundered the poor until they were forced to revolt. Therefore, to Dickens, oppression on a large scale results in anarchy. Having a strong conviction that England paralleled France, Dickens was concerned that the English people might erupt at any moment into a mass of bloody revolution, as well. The dissastisfaction of industrial workers in the 1830s in England did reach alarming national proportions with the Chartist movement, and the unemployed were always a threat. This conviction is what became the occasion for his serious and powerful work of art.
It may seem to the naked eye that Dickens favors the upper classes, but what he is really doing is inflating them as a technique to be able to deflate them later.
A Tale of Two Cities explores the excess and debauchery that existed prior to the French Revolution. To be exact, the years prior to it were truly ridiculous as far as the pomp and ceremony in France with Marie Antoinette and Louis XVI and eventually in England during the regency of George III's son, the future George IV.
The life in Versailles, or "in court" was excessive, arrogant, glittery, and financially unsound while life outside of court, the slum districts, and the poor were truly miserable (Hence, the story "Les Miserables"). The aristocracy truly turned a blind eye on what was going on, and their vacuous lives were frivolous, backstabbing, and superficial.
The best technique to contrast life before and after the revolution would then be to elevate the aristocracy in print the same way that they elevated themselves, so that the fall from grace is all the more dramatic towards the conclusion.
I am not sure I fully agree with herappleness: clearly Dickens, who had experienced poverty in his youth as a result of his father's imprisonment for debt, was sensitive to the plight of the poor. Indeed, he championed the cause of the poor throughout his life and repeatedly pointed out the atrocities of the system through his novels.
While fully aware that the poor can be as mean and heartless as the rich, Dickens did sympathize nevertheless with the poor, who, in his view, have more merit in being good than the rich. For instance in “The Haves and the Haves not” (American notes, 1842), he remarks:
“Cant as we may, and as we shall to the end of all things, it is very much harder for the poor to be virtuous than it is for the rich; and the good that is in them, shines the brighter for it.”