MEXICO CITY - Mexico's ruling coalition fell just one senator short of securing a supermajority in the Senate on Wednesday after two opposition senators flipped, according to President-elect Claudia Sheinbaum.

The two-thirds supermajority needed to alter the Constitution opens the door to a controversial judicial reform which has roiled markets and heightened tensions between Mexico and its northern neighbors, the U.S. and Canada, in recent days.

The Morena ruling party and its allies already locked in a supermajority in the lower house for the congressional session set to start in September, the electoral authority confirmed last week.

Two senators from the center-left Democratic Revolution Party (PRD), Araceli Saucedo and Jose Sabino Herrera, renounced their party affiliation and joined Morena. While ideologically similar to Morena, the PRD was part of an opposition coalition with more conservative parties in the June elections.

"I welcome two extraordinary colleagues who made the historic decision to join our movement's legislative bloc," Sheinbaum said at a meeting with Morena party members. "There will be 85 senators from our movement," she added.

Following the announcement, the Mexican peso trimmed gains from earlier in the morning, appreciating just 0.36% against the U.S. dollar.

Congress is set to convene on Sunday and in the coming days will begin debate on a series of constitutional reforms proposed by outgoing President Andres Manuel Lopez Obrador.

His bid to overhaul the judicial system to allow the popular election of judges - including Supreme Court judges - has prompted investor jitters and expressions of concern from the U.S. and Canadian embassies.

Proponents say the reform will boost democracy and help fix a system they argue does not serve the public, while critics say it will skew power in favor of the executive, cut off judges' careers, and make the courts more susceptible to criminal influence.

  - Reuters