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The mobility μ of excess electrons in dense Argon gas was measured as a function of the applied electric field E and of the gas density N at several temperatures in the range 142.6 < T < 200 K, encompassing the critical temperature Tc = 150.86 K We report here measurements up to N ≈ 7 nm-3, close to the critical density, Nc ≈ 8.1 nm-3. At all temperatures, and up to moderately high densities, the density-normalized mobility μN shows the usual electric field dependence in a gas with a Ramsauer-Townsend minimum due to the mainly attractive electron-atom interaction. μN is constant and field independent for small E, shows a maximum for a reduced field E/N ≈ 4 mTd, and then decreases rapidly with the field. The zero field density-normalized mobility μ0N, for all T > Tc, shows the well known anomalous positive density effect, i.e., μ0N increases with increasing N. Below T,, however, μ0N does not show the expected effect, but features a broad maximum. This appears to be a crossover behavior between the positive density effect shown for T > T, and the small negative effect previously observed for T ≈ 90 K However, the data at all temperatures confirm the interpretation of the anomalous density effect as being essentially due by the density-dependent quantum shift of the electron ground state kinetic energy in a disordered medium as a result of multiple scattering (MS) processes, although other MS processes influence the experimental outcome.