MEXICO CITY—The majority obtained in Mexico’s Congress by the political parties supporting Andrés Manuel López Obrador, the winner of Sunday’s election, is so overwhelming that the future president is very close to the two-thirds majority needed to pass constitutional reforms.
Mr. López Obrador’s broad coalition, formed by his own Movement for National Regeneration, the far-left Labor Party and the conservative Social Encounter party, got 307 out of 500 representatives in the Lower House and 68 out of 128 senators, according...