We propose a multi-year optical observational campaign to characterize the activity and nuclei of a number of known main-belt comets (MBCs) and near-Earth MBC (NEMBC) candidates. MBCs are rare objects that are dynamically indistinguishable from main-belt asteroids, yet display cometary activity indicative of sublimating ice, while NEMBCs are Jupiter-family comets recently identified as potentially originating in the asteroid belt, and which therefore represent exciting potential opportunities to probe icy main-belt objects at close proximity. Our objectives are to monitor and analyze the activity of 10 MBCs expected to be active between 2019 and 2022 to constrain onset and termination times and monitor activity strength evolution, and conduct physical characterization observations of 11 inactive MBC nuclei and 6 inactive NEMBC candidate nuclei to determine phase functions and compositional properties. Results of our analyses will be used to study long-term activity attenuation in MBCs, improve our understanding of volatile preservation in asteroids and the likely total fraction of icy objects in the asteroid belt, and assess the likelihood that NEMBC candidates originated in the asteroid belt. This research is supported by a funded NASA Solar System Observations program (PI: H. Hsieh) for which Gemini observations constitute a central component.