Chabad messianism, or Lubavitch messianism, generally refers to the passion among adherents of the Chabad movement regarding the coming of the Messiah and their goal to raise awareness that his arrival is imminent. In addition, the term also refers more specifically to the hope that Rabbi Menachem Mendel Schneerson could himself be the Messiah.
The concept that the Messiah can come at any given moment is a basic tenet of Judaism. The idea that the leader of a Hasidic group could be the Messiah is a nuanced and complex idea that traces itself back to the times of the Baal Shem Tov, the founder of Hasidism. During Rabbi Schneerson’s life many Jews hoped that he would be the Messiah. This idea gained great vocal attention during the last years of Schneerson life. During those years, there were strong forces within the Chabad–Lubavitch movement, led by the two executors of Schneerson's will; Rabbi Yehuda Krinsky, and Rabbi Abraham Shemtov, resisting the messianic movement.
Since the Rebbe's death, there are those who have persisted in the belief that Schneersons could still be the Messiah, sometimes extremely vocally. Chabad messianists (meshichists) refuse to admit that Schneerson died, instead proclaiming that he is still alive. The Chabad–Lubavitch umbrella organization, Agudas Chasidei Chabad, and the governing body of Chabad–Lubavitch rabbis, Vaad Rabonei Lubavitch, have both denounced the messianic behavior. The issue received much negative attention. To this day it remains controversial.
During Menachem M. Schneerson's life some of his supporters thought that he could be revealed as the Messiah. There were many Chabad Hasidim who proclaimed this very passionately. This concept, that a righteous Jewish leader may be revealed as the Messiah, was believed by many rabbis and scholars regarding their own leaders throughout Jewish history.
This idea finds its earliest roots in an early 4th century discussion in the Talmud: After schools of various prominent rabbis each cite proof-texts that the name of the Messiah is indeed that of their very own teacher, Rabbi Nachman says, "If he [the Messiah] is among the living, he is someone like me." The Talmud then cites the sage Rav: "If the Messiah is among the living, then he is someone like our holy Rabbi [i.e., Rabbi Judah, the editor of the Mishnah]. [And] if he is from among the dead, he is someone like Daniel."