The nonenzymatic polymerization of RNA may have enabled copying of functional sequences during the origin of life. Recent progress utilizing 5'-phosphoro-2-aminoimidazole activation has reinvigorated the possibility of using nonenzymatic RNA polymerization for copying arbitrary sequences. However, the reasons why 2-aminoimidazole (AI) is a superior activation group remain unclear. Here we report that the predominant mechanism of polymerization using cytidine-5'-phosphoro-2-aminoimidazolide (Cp*) involves a 2-aminoimidazolium-bridged dinucleotide (Cp*pC) intermediate. To explore the role of this intermediate, we first identify and quantify four reactions involving the synthesis and breakdown of Cp*pC that occur in the absence of the primer-template duplex. We then analyze the dependence of the rate of polymerization on the concentration of the Cp*pC intermediate in the presence and absence of the competitive inhibitor Cp. We also show that the contribution of the monomer Cp* to the polymerization rate is negligible under our primer extension conditions. Finally, we use the experimentally determined rate constants of these reactions to develop a kinetic model that helps explain the changing rate of nonenzymatic RNA polymerization over time. Our model accounts for the concentration of Cp*pC formed by Cp* under primer extension conditions. The model does not completely account for the decline in polymerization rate observed over long times, which indicates that additional important inhibitory processes have not yet been identified. Our results suggest that the superiority of 2-aminoimidazole over the traditional 2-methylimidazole activation is mostly due to the higher level of accumulation of the imidazolium-bridged intermediate under primer extension conditions.