Final answer:
The new political party formed in Mexico in 1921 was the Institutional Revolutionary Party (PRI), which dominated the political landscape well into the twenty-first century and led Mexico through significant social and economic reforms.
Step-by-step explanation:
In 1921, Mexico saw the emergence of a new political party, the Institutional Revolutionary Party or PRI. After the Mexican Revolution that began in 1910 and the subsequent series of revolutionary generals leading the nation without a clear line of presidential succession, the formation of the PRI aimed to stabilize and structure Mexican politics. The PRI, despite various name changes over time, remained the dominant force in Mexican politics well into the twenty-first century. Although there were regular elections and an expansion of voting rights suggesting a movement towards democratic institutions, the PRI effectively controlled a one-party state. All presidents and nearly every member of the legislature came from the PRI. It wasn't until 1938 that President Lázaro Cárdenas would enact more radical changes, supporting labor unions, nationalizing the oil industry, and championing land reform and socialist policies.