One simple figure of speech that Johnson uses is the metaphor of the support that the North Vietnamese are sending to support their communist fighters in the south as "the heartbeat of the war." The idea is that without this constant flow of support, the war would quickly and simply expire.
Johnson also uses rhetorical questions, namely "Why are these realities our concern? Why are we in South Viet-Nam?" The second portion of his speech, where he makes the main arguments for the continued escalation of the war, are the answers to these rhetorical questions.
He also uses anaphora at the end of the speech to emphasize the reasons for why the United States is continuing involvement in South Vietnam. He repeats "we do this" three times to outline exactly why it is that we continue to send support to Vietnam.