Is the prohibition of Melacha on Chol Hamoed Biblical or Rabbinical?
It is disputed amongst Poskim whether the prohibition of doing Melacha on Chol Hamoed is of Biblical or Rabbinical origin. Some Poskim rule it is Biblically forbidden. Other Poskim rule it is Rabbinically forbidden. Practically, Admur rules it is merely of Rabinically forbidden. Nevertheless, the Sages stated that one who belittles the Moed does not have a portion in the world to come and it is considered as if he has served idols. Hence, one must be very careful in the laws of the Moed, even though they are considered merely Rabbinical in nature.
 Rashi; Rashbam; Ramban; Ritva; Rabosav of Yireim; Meiri in name of many Geonim; Rif; Sefer Hachinuch; Shivlei Haleket; Eshkol; Ritz Geius.
 Rabbeinu Tam; Riva; Rambam; Smag; Bahag; Rosh; Nimukei Yosef; Riaz; Meiri; Rid; Rabbeinu Gershom; Orchos Chaim; Tashbatz; Tur; Kol Bo; Shulchan Aruch Harav