Trump’s lawyers argued the gag order was stifling his campaign speeches and said it might limit his ability to respond to attacks from Mr Biden during the debate.
“There is ample evidence to justify continued concern for the jurors,” Mr Merchan wrote.
“Until a sentence is imposed, all individuals covered… must continue to perform their lawful duties free from threats, intimidation, harassment and harm,” he added.
The Trump campaign saw a major boost in donations after his conviction on May 30, and has since used the court cases against him as evidence that he is the victim of a political conspiracy.
Three more cases against him, for alleged election interference and mishandling classified documents, are pending in Washington, Atlanta and Miami.