Letters suggesting the Trump administration tried to discourage a former acting U.S. attorney general from testifying before Congress, in a hearing that ended up being canceled, are likely to further inflame tensions between the parties over whether the White House has been forthright regarding alleged Russian interference in the 2016 election.

Sally Yates, a deputy attorney general under Barack Obama who was acting attorney general in the early days of the Trump administration, was to testify Tuesday along with former Central...