Former Trump officials back Biden, saying it is time to put ‘country over party.’

“I haven’t read her comments in any detail,” Mr. Pence said. “But it reads to me like one more disgruntled employee that has decided to play politics during election year. My staff has indicated and she made no comments like that when she was serving under our team here at the White House.”

“I couldn’t be more proud of the work we’ve done,” the vice president added.

Lt. Gen. Keith Kellogg, the national security adviser to Mr. Pence, said Ms. Troye never expressed “any concern regarding the administration’s response to the coronavirus to anyone in her chain of command.”

Ms. Troye, who played a central role in running the White House’s coronavirus task force until leaving the government last month, is one of two top Trump administration officials who announced their opposition to Mr. Trump on Thursday and joined more than two dozen other Republican officials as part of a new group calling for “leadership change in the White House.”

Josh Venable, who served as chief of staff for Betsy DeVos, the secretary of education, will also join the group, known as REPAIR (Republican Political Alliance for Integrity and Reform), according to a statement. John Mitnick, a former top lawyer at Mr. Trump’s Department of Homeland Security, is an adviser to the group.

Ms. Troye, a lifelong Republican who also worked in the George W. Bush administration, said in the statement that she would serve as a founder of the group, with the goal of restoring integrity to the Republican Party.