Painting overtly political subjects can be a risky task. There's the obvious risk of supporting a side that eventually loses -- will the winners seek retribution? But another risk is that, once an issue is no longer current, the paintings will be forgotten and the artist as well. Which might be why political paintings represent a rare genre.
Another consideration related to transitory political issues is painting technology. While a painting might take weeks or even months to complete, posters can be on the streets in a matter of days from when an inspiration strikes. This is why most political art is in poster form.
Before the 19th century most art was commissioned by the church, state, and rich or powerful individuals. Political content, such as it was, therefore was mostly in support of those who hired the artist. That is, anti-establishment painting subjects were rare because they were seldom funded. As painters became less reliant on traditional commission sources, they became increasingly able to create critical art.
As is noted here, Delacroix's famous work commemorates a successful regime change even though it appears to be a call to arms. The new regime (that of Louis-Philippe) eventually entered history's dustbin, but the huge painting lives on in the Louvre.
The Russian revolution is glorified and a Mexican version encouraged in this mural that even includes Rivera's occasional wife Frida Kahlo as the central subject. Mexican governments at the time regarded themselves as "revolutionary" and tolerated such themes for murals on public buildings.
Peter Blume (1906-92) - completed 1937
I wonder how many people today would be able the grasp the context of this painting if they encountered it at New York's Museum of Modern Art and saw a plaque containing only the information above. The subject is anti-Fascism and the green jack-in-the-box figure represents Italian dictator Benito Mussolini. No one under age 70 can remember the living Mussolini who is increasing an historical footnote.
This looks like a computer-generated "painting" (note the treatment of the plane's engine cowling) though in principle it could have been rendered in oil, acrylic or gouache. Like Blume's painting, it is likely to age poorly because it isn't very interesting artistically and deals with an ephemeral subject requiring specialized knowledge (i.e., Sarah Palin likes to hunt).
Here is the sort of political-themed "art poster" common since the mid-1960s and reaching a climax during the presidency of George W. Bush (shown). Many current political-themed posters make use of computer-manipulated collages such as seen here; it's a quick way to create something with visual interest and realistic looking detail. Were this a painting, I doubt it or its artist would be long-remembered. It's too topical and historically questionable.