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Nebular-phase observations of peculiar Type Ia supernovae (SNe Ia) provideimportant constraints on progenitor scenarios and explosion dynamics for boththese rare SNe and the more common, cosmologically useful SNe Ia. We presentobservations from an extensive ground-based and space-based follow-up campaignto characterize SN 2022pul, a "super-Chandrasekhar" mass SN Ia (alternatively"03fg-like" SN), from before peak brightness to well into the nebular phaseacross optical to mid-infrared (MIR) wavelengths. The early rise of the lightcurve is atypical, exhibiting two distinct components, consistent with SN Iaejecta interacting with dense carbon-oxygen rich circumstellar material (CSM).In the optical, SN 2022pul is most similar to SN 2012dn, having a low estimatedpeak luminosity ($M_{B}=-18.9$ mag) and high photospheric velocity relative toother 03fg-like SNe. In the nebular phase, SN 2022pul adds to the increasingdiversity of the 03fg-like subclass. From 168 to 336 days after peak $B$-bandbrightness, SN 2022pul exhibits asymmetric and narrow emission from [O I]$\lambda\lambda 6300,\ 6364$ (${\rm FWHM} \approx 2{,}000$ km s$^{-1}$),strong, broad emission from [Ca II] $\lambda\lambda 7291,\ 7323$ (${\rm FWHM}\approx 7{,}300$ km s$^{-1}$), and a rapid Fe III to Fe II ionization change.Finally, we present the first-ever optical-to-mid-infrared (MIR) nebularspectrum of an 03fg-like SN Ia using data from JWST. In the MIR, strong linesof neon and argon, weak emission from stable nickel, and strong thermal dustemission (with $T \approx 500$ K), combined with prominent [O I] in theoptical, suggest that SN 2022pul was produced by a white dwarf merger withincarbon/oxygen-rich CSM.

Ground-based and JWST Observations of SN 2022pul: I. Unusual Signatures  of Carbon, Oxygen, and Circumstellar Interaction in a Peculiar Type Ia  Supernova