Boris Johnson, bracing for possible Biden presidency, defends Brexit plan.

Though Joseph R. Biden Jr. holds no official government position, the Democratic nominee is fast becoming a factor in calculations by foreign leaders eyeing the possibility of a return to pre-Trump diplomatic norms.

Case in point: Boris Johnson, the British prime minister, was forced to respond to concerns from Mr. Biden that rewriting the United Kingdom’s withdrawal agreement with the European Union could imperil the Good Friday Agreement, which ended decades of bloody conflict in Northern Ireland.

Congressional Democrats have warned that the landmark pact, signed in 1998, could be weakened as Britain works out international agreements that have been impacted by Mr. Johnson’s manifold Brexit maneuvers.

Mr. Johnson’s plan to override a landmark agreement with the European Union, a move his government has admitted would violate international law, has brought potential repercussions to a possible trade agreement with the United States.