The biggest “super PAC” in the presidential race is opening up a last-minute burst of advertising on Hillary Clinton’s behalf, seeking to bolster her in three hotly contested states as she fends off renewed questions about her private email server.

The group, Priorities USA Action, is spreading $6 million across nine television markets in three states, according to a Democratic media buyer. Some of the spending will occur in Wisconsin, a state where the group had not previously spent any money this election and where Donald J. Trump is running close to Mrs. Clinton.