In the old days, book distributors and sankirtan devotees usually carried some mantra cards like the one above to distribute. Hopefully this practice is still going on, because if there was only one piece of literature being distributed, it should be the maha- mantra, or the great mantra for deliverance.
Following in the footsteps of Lord Caitanya Mahāprabhu, the Kṛṣṇa consciousness movement is distributing the Hare Kṛṣṇa mahā-mantra and inducing people all over the world to chant. We are giving people an immense treasury of transcendental literature, translated into all the important languages of the world, and by the grace of Lord Śrī Caitanya Mahāprabhu this literature is selling profusely, and people are chanting the Hare Kṛṣṇa mahā-mantra with great delight. This is the preaching process of the Caitanya cult. Since the Lord wanted this cult preached all over the world, the International Society for Krishna Consciousness is acting in a humble way so that the vision of Śrī Caitanya Mahāprabhu may be fulfilled all over the world, especially in the Western countries. (from purport to Adi-lila, 6.19)
This is the sublime mission of the International Society for Krishna Consciousness. Many people come and inquire whether they have to give up family life to join the Society, but that is not our mission. One can remain comfortably in his residence. We simply request everyone to chant the mahā-mantra: Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare. If one is a little literate and can read the Bhagavad-gītā As It Is and Śrīmad-Bhāgavatam, that is so much the better. These works are now available in an English translation and are done very authoritatively to appeal to all classes of men. Instead of living engrossed in material activities, people throughout the world should take advantage of this movement and chant the Hare Kṛṣṇa mahā-mantra at home with their families. One should also refrain from sinful activities-illicit sex, meat-eating, gambling and intoxication. (from purport Madhya-lila 7.128)
Purport to Hare Krishna Maha Mantra
Los Angeles, July 10, 1971
Prabhupāda: So this mahā-mantra, Hare Kṛṣṇa Hare Kṛṣṇa Kṛṣṇa Kṛṣṇa Hare Hare, is being chanted by Brahmā with his four mouths. Brahmā-jape catur-mukhe. Brahmā means Lord Brahmā, and jape means he is chanting, catur-mukhe, with his four heads. Within this universe, only Brahmā has got four heads. And Lord Śiva sometimes exhibits five heads. So it is explained here that Lord Brahmā is also chanting this Hare Kṛṣṇa Hare Kṛṣṇa Kṛṣṇa Kṛṣṇa Hare Hare, Hare Rāma Hare Rāma Rāma Rāma Hare Hare, with his four mouths. Still he is chanting. He does not think that he is satiated. This transcendental name is so sweet that either you chant with your one mouth and one tongue or with one thousand mouths or one thousand tongues, still, you will never feel tired. That is the purport of this song. Śrīla Rūpa Gosvāmī, he lamented that “This Hare Kṛṣṇa Hare Kṛṣṇa Kṛṣṇa Kṛṣṇa Hare Hare, Hare Rāma Hare Rāma Rāma Rāma Hare Hare, these names are so sweet that how I can relish the transcendental sweetness with one tongue and one mouth? If God would have given me millions of tongues and millions of mouths, then I would have relished a little of it.” So he lamented. Here also it is said that Nārada Muni, he has got always with him a tampura, and he is traveling all over the universe constantly. He cannot stay in any place more than a few seconds, and he has no companion. But his only companion is that tampura and chanting Hare Kṛṣṇa. Therefore it is said here, nārada-yabe, vīṇā-yantre, kṛṣṇa kṛṣṇa hare hare. (end)
The Hare Krishna Movement 