Context. Gaining a full understanding of the planet and moon formation process calls for observations that probe the circumplanetary environment of accreting giant planets. The mid-infrared ELT imager and spectrograph (METIS) will provide a unique capability to detect warm-gas emission lines from circumplanetary disks. Aims. We aim to demonstrate the capability of the METIS instrument on the Extremely Large Telescope (ELT) to detect circumplanetary disks (CPDs) with fundamental v = 1 0 transitions of 12CO from 4.5 to 5 μm. Methods. We considered the case of the well-studied HD 100546 pre-transitional disk to inform our disk modeling approach. We used the radiation-thermochemical disk modeling code ProDiMo to produce synthetic spectral channel maps. The observational simulator SimMETIS was employed to produce realistic data products with the integral field spectroscopic (IFU) mode. Results. The detectability of the CPD depends strongly on the level of external irradiation and the physical extent of the disk, favoring massive (∼10 MJ) planets and spatially extended disks, with radii approaching the planetary Hill radius. The majority of 12CO line emission originates from the outer disk surface and, thus, the CO line profiles are centrally peaked. The planetary luminosity does not contribute significantly to exciting disk gas line emission. If CPDs are dust-depleted, the 12CO line emission is enhanced as external radiation can penetrate deeper into the line emitting region. Conclusions. UV-bright star systems with pre-transitional disks are ideal candidates to search for CO-emitting CPDs with ELT/METIS. METIS will be able to detect a variety of circumplanetary disks via their fundamental 12CO ro-vibrational line emission in only 60 s of total detector integration time. @en