The need for a constitutional convention was brought home to Andrew Jackson, the co-Governor of Jefferson, when word reached him of a coup attempt in Mexico City by supporters of the late Federalist leader José María Morelos on 1 February 1820. Although the coup attempt was put down with ease by Jeffersonian soldiers under Barton Kelly, it became clear to Jackson that relations between Jefferson and Mexico must be "made more reasonable than they are at present." Jackson left Jefferson City for Mexico City, arriving there on 4 May and resuming his duties as provisional president of Mexico. Co-Governor Josephus Carter addressed the Jeffersonian Chamber of Representatives the following month, relaying Jackson's request that a constitutional convention be held in Mexico City to effect a permanent union of Jefferson and Mexico. On 15 June the Chamber voted to adjourn itself and re-assemble in Mexico City on 22 September as a constitutional convention.
In Mexico City, the Jeffersonians found themselves joined by Jackson's followers among the Clericalists, as well as non-voting delegates from the Indian tribes and observers from other Mexican factions. After six days, the convention was addressed by Jackson, in the course of which he outlined the basic features of the Mexico City Constitution: Jefferson would be one of six Mexican states governed by a national government based in Mexico City. The national government would control all taxation and military units, and would be governed by a strong executive along with a bicameral legislature and an independent judiciary. At a dinner that night, Jackson announced that the new nation would be called the United States of Mexico.
Despite opposition from the Liberty Party among the Jeffersonians and the non-voting delegates, the delegates ratified Jackson's proposed constitution with only minor changes. One year later, elections were held under the new constitution, and the new government, led by Jackson, was established.
Sobel's source for the Mexico City Convention is Pedro Cordovan's Jackson at the Convention: The Strokes of Genius (Mexico City, 1962).