Mexico’s Leftists Won Big. Investors Are Worried.

A final count of votes released over the weekend suggests Mexico’s leftist governing party and its allies would capture large majorities in Congress, potentially enabling the coalition to pass sweeping changes to the Constitution.

The official tally from elections last week showed that the party, Morena, and its partners appeared on their way to clinching a two-thirds supermajority in the lower house of Congress.

In the Senate, it seemed that the coalition would fall short of a supermajority — but by a small number of seats, analysts said, meaning it would likely need to attract the support of only a few opposition legislators to alter the Constitution. Building those alliances “is relatively easy to achieve,” said the party’s president, Mario Delgado, in an interview.

“We are now a dominant force,” Mr. Delgado added, “by the decision of the people.”

The final makeup of the legislature is still unclear because a share of seats in the Mexican Congress are appointed via a system of proportional representation in August. Legal challenges could also affect how seats are allocated.

But Morena has come close enough to total dominance to prompt a strong reaction from a sector that the party can’t ignore: the financial markets.

In the volatile days following the election, investors’ alarm has been on full display, with Mexican stocks battered and the peso suffering its worst week since the pandemic.