Throughout the year, Trump had begged to debate Biden with complete indifference to the details. However, the former president and his team became reluctant to face Kamala Harris when she became the Democratic candidate. Trump is now backpedaling out of fear over how he would perform against a younger, more dynamic figure like Harris.
It doesn't matter how hard Democrats and their media allies try to act as if Trump is dodging the September debate; that claim will never be valid. He had agreed to face Biden twice after details were negotiated between their teams, so new negotiations had to occur after Harris' nomination. The Vice President is looking for a way out of risking defeat in a debate against Trump.