MEXICO CITY — With just weeks left in his term, President Andrés Manuel López Obrador is trying to ram major changes to the constitution through Mexico’s Congress in a parting shot that diplomats say threatens Mexico’s young democracy and the United States’ most important trade relationship.

       A newly elected Chamber of Deputies appears ready to vote to abolish the current judicial system as early as next week, with the Senate to follow. Under López Obrador’s proposal, citizens would elect nearly all federal judges — including those on the Supreme Court.