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The BL Lac object 3C 66A was the target of an extensive multiwavelengthmonitoring campaign from July 2003 through April 2004, involving observationsthroughout the electromagnetic spectrum. Radio, infrared, and opticalobservations were carried out by the WEBT-ENIGMA collaboration. At higherenergies, 3C 66A was observed in X-rays (RXTE), and at very-high-energy (VHE)gamma-rays (STACEE, VERITAS). In addition, the source has been observed withthe VLBA at 9 epochs throughout the period September 2003 - December 2004,including 3 epochs contemporaneous with the core campaign.The WEBT campaignrevealed microvariability with flux changes of ~ 5 % on time scales as short as\~ 2 hr. The source was in a relatively bright state, with several brightflares on time scales of several days. The SED indicates a $\nu F_{\nu}$ peakin the optical regime. A weak trend of optical spectral hysteresis with a trendof spectral softening throughout both the rising and decaying phases, has beenfound. The 3 - 10 keV X-ray flux of 3C 66A during the core campaign washistorically high and its spectrum very soft, indicating that the low-frequencycomponent of the broadband SED extends beyond ~ 10 keV. No significant X-rayflux and/or spectral variability was detected. STACEE and Whipple observationsprovided upper flux limits at > 150 GeV and > 390 GeV, respectively. The 22 and43 GHz data from the 3 VLBA epochs between September 2003 and January 2004indicate a smooth jet with only moderate internal structure. Evidence forsuperluminal motion (8.5 +/- 5.6 h^{-1} c) was found in only one out of 6components. The radial radio brightness profile suggests a magnetic field decay$\propto r^{-1}$ and, thus, a predominantly perpendicular magnetic field.Comment: 27 pages, including 17 figures. Accepted for publication in Ap

Coordinated Multiwavelength Observation of 3C 66A during the WEBT Campaign of 2003–2004