We compare four explanations of transpolar potential saturation: (1) the magnetic field at the stagnation point weakens, thereby limiting magnetic reconnection; (2) a dimple develops at the stagnation point, which limits the inflow rate to the reconnection line; (3) the magnetopause becomes blunt and the bow shock recedes, thus giving more room for the solar wind to flow around the magnetosphere, thereby reducing the need for magnetic reconnection; (4) the region 1 current system usurps the Chapman-Ferraro current system and saturates when the J ¿ B force it generates balances solar wind ram pressure. The paper's point is that all four mechanisms involve a limit on the strength of the region 1 current system and that the criterion for the onset of transpolar potential saturation in each mechanism is that the region 1 current system generates a magnetic field that is about as strong as the dipole field at the dayside magnetopause. This circumstance prevents tests to discriminate between the four mechanisms based on predictions that relate to their dependencies on the region 1 current system. The group as a whole, however, can be tested to see whether their common criterion that relates the onset of transpolar potential saturation to the total current flowing in the region 1 system holds. The criterion can be formulated in terms of predictions that relate transpolar potential saturation to the strength of the interplanetary electric field, solar wind ram pressure, and ionospheric conductance. Published data analyses and MHD simulations reasonably confirm these predictions.