In Tom Hiddlestone's Coriolanus in 2011, the story of the eponymous tragic hero is set in the years just before the fall of the Republic (as opposed to when it's traditionally set, in the 5th century BC).
The play starts with lots of political graffiti. As famine threatens the city, demands for bread and citizens criticising the ruling elites are drawn against the wall.
On the one hand, the Roman elite were not hugely popular with the plebeians in the 1st century BC. On the other hand, I also imagine they were mostly illiterate, so would not be writing anything on the walls.
Was there much political graffiti at all in Ancient Rome? Is it reasonable to imagine things like "Give us Bread" and "I am the Down with the Senate" written on walls and homes?
Firstly I think it's important to point out that Rome's social hierarchy wasn't as simple as the educated nobilitas versus unwashed uneducated plebs. Although literacy amongst the lowest and largest tier of society was low, there was a thriving middle class of people, both who had standing in society (The equites) and just people with a bit more wealth to their names (artisans, traders, etc.), who would have educated their sons to at least have basic literacy.
As for the graffiti itself, yes, tonnes of it. Brutus, for example, cited graffiti on the walls of Rome as one of the reasons for joining the anti-Caesar conspiracy, with one of the messages being "Brutus, are you sleeping?"
Pompeii, unsurprisingly, has been a great source for it. Here's some examples of explicitly political graffiti:
-Caium Iulium Priscum IIvirum - Gaius Iulius Priscus for magistrate
-Caium Iulium Polybium duumvirum iure dicundo dignum rei publicae - Gaius Iulius Polybius for magistrate, he is worthy of public office
-Lucium Statium Receptum IIvirum iure dicundo oro vos faciatis vicini dignum scribsit Aemilius Celer viciniinvidiose qui deles aegrotes - Your neighbours beg you to vote for Lucius Statius Receptus, who is a worthy man, as magistrate with political power. Your fellow neighbour Aemilius Celer wrote this. May any jealous person who removes it fall ill.
Finally ,even though not technically political I do want to share some my favourite Pompeii graffiti in general: "Weep, you girls. My penis has given you up. Now it penetrates men’s behinds. Goodbye, wondrous femininity!". This in tandem with all the big cocks they drew shows that humanity never changes.