Writing to convince. Dos Passos' U.S.A., Aragon's The Communists

What strikes by reading Dos Passos' trilogy U.S.A. and Aragon's Communists, is their power of persuasion. What strikes by studying them, is the lack, or even the refusal of an explicit militant speech. How to structure a committed speech about the real world, how to convince the reader from some views the authors hold for more valuable than others, into novels which won't follow the example of the "tendenz novels"? The answer of both our works is the same: It is necessary to write literature, and not propaganda. The abandon of the usual militant tricks appears as a pragmatic necessity into a convincing strategy. What are its techniques, what is its inside?