Over the past seven years, a leftist political party called Morena has accomplished a remarkable takeover of the Mexican political landscape.

It has elected two consecutive presidents, secured supermajorities in Congress, made sweeping political moves that cemented its authority and left the opposition so badly beaten that it is clinging to life.

Now, Morena could take one of the most important steps yet in its consolidation of power. On Sunday, Mexicans will head to the polls to elect every federal judge in the nation and many local ones — 2,682 justices, judges and magistrates in all — a first-in-the-nation vote to overhaul the judiciary.

Morena leaders said they decided on the election to fix a justice system rife with corrupt judges who served the elite, rather than everyone, and who kept frustrating the party’s plans. In the process, they could eliminate the final major check on Morena’s power.

Many legal and political analysts in Mexico expect candidates aligned with Morena to dominate the election, filling judgeships from local courthouses to the Supreme Court and giving the party effective control over the third branch of government.