Democrats, teachers’ unions and liberal protesters have been flooding senators with calls and emails protesting her nomination for myriad reasons. They have expressed concern about her family’s contributions to groups that support so-called conversion therapy for gay people; her donations to Republicans and their causes, which she agreed have totaled about $200 million over the years; and her past statements that government “sucks” and that public schools are a “dead end.”

Opponents have focused on the poor performance of charter schools in Detroit, which she has bankrolled even as she resisted legislation that would have blocked chronically failing charter schools from expanding.

Senator Patty Murray of Washington, the ranking Democrat on the committee, insisted she needed more time to question Ms. DeVos about possible conflicts of interests and about her dedication to ensuring the protection of public schools, which she eschewed for her own children.

“I have not been persuaded that Betsy DeVos will put students first if she were confirmed, and I have not been persuaded that she has the experience, skills, understanding, or vision to lead this critical department at a time when it is more important than ever,” Ms. Murray said. “From everything we heard, everything we know, and all of the questions that still remain, it is clear to me that Betsy DeVos is the wrong choice to lead our nation’s Department of Education.”