MEXICO CITY—President Andrés Manuel López Obrador’s party and its allies on Monday appeared poised to maintain their majority in Mexico’s lower chamber of the congress, but fell short of a two-thirds majority as some voters boosted the opposition, according to initial election results. Electoral authorities released “quick count” results based on voting samples that allow estimates of the voting trends to determine the rough potential makeup of the Chamber of Deputies late Sunday. López Obrador’s Morena party will have to rely on votes from its allies in the Workers Party and Green Party, but together they were expected to capture between 265 and 292 seats in the 500-seat chamber. Morena alone was expected to win 190 to 203 seats. That would signal a significant decline for the president’s party. In the current congress, Morena has a simple majority, holding 253 seats on its own. It would also deprive the president …