One of the predominant themes dealt with in Barchester Towers is that of politics, particularly the idea of a community that is to a large degree composed of conservatives should suddenly come under the power of liberal (Whig) outsiders; in this case Slope and the Proudies. It is not a novel theme in itself, but is interesting in terms of Trollope's treatment of it in the context of the Church of England, which was at the time reacting to a reform movement, and also in the context of the contemporary rise of the upper middle classes.
This theme of change instigated from outside is present on all levels of the novel and is brought into play right from the start with the death of old Dr. Grantly and the fall of the conservative government. Trollope uses the microcosm of the Barchester clergy and those near to them to explore the nature of political competition. One sees how characters are drawn to side with one party or another and how those, such as the Stanhopes, who are not especially of either conviction, are courted by both sides for their support. The Proudie faction, while Dr. Proudie, his wife and Slope are united, is the more powerful of the two sides, but as soon as the struggle for power between the three of them begins in earnest they are divided and weakened. The Grantly faction too is divided on account of a lack of communication within itself; in this respect one thinks particularly of the suspicions surrounding Eleanor Bold's relationship with Slope, Harding's weak character and Arabin's distraction on account of personal doubts and concerns. However, they win in the end not so much as a result of their own organisation, but more because of the more serious problems in the Proudie camp. It is therefore more a question of who loses than who wins in this particular political skirmish.
Finally, Trollope also examines social politics in his treatment of the Thornes and their Fete Champetre. They are represented as the last bastion of a die-hard conservatism and traditionalism that can only be appreciated in the rosy light of nostalgia, and which has little value in the modern world. Trollope also shows in the Thornes' attempts at social division into guests of "quality" and otherwise, and in how the reinstitution of old-fashioned games can be dangerous, a couple of the more ugly aspects of the antique social philosophy that the Thornes represent.