Tina Tchen, who worked for eight years in the White House, mostly as Michelle Obama's chief of staff, said last week the toughest part of the job has been managing the many requests for the first lady's time. The deluge of invitations is not likely to let up after the Obamas leave the White House, said Anita McBride, who was chief of staff to first lady Laura Bush and is now an executive-in-residence at the center Thurber leads at American University.