Following is the e-mail she sent the campaign at 3:44 p.m. ET:
Over a year ago Hillary launched her campaign for President.
Her announcement began a historic effort that has inspired millions and brought hundreds of thousands to their feet all across this nation.
I have been proud to manage this campaign, and prouder still to call Hillary my friend for more than sixteen years. I know that she will make a great President.
This has already been the longest Presidential campaign in the history of our nation, and one that has required enormous sacrifices from all of us and our families.
During the last month I have been working closely with my longtime friend, Maggie Williams.
This week Maggie will begin to assume the duties of campaign manager. I will serve as a senior adviser to Hillary and the campaign and travel with Hillary from time to time on the road. Maggie is a remarkable person and I am confident that she will do a fabulous job.
Although I will continue to see you all at headquarters, I would be remiss if I didn't thank each of you for your dedication, excellence, and passion over the last year.
You are the best campaign staff in the history of Presidential politics and I am grateful to each of you for your hard work and friendship.
ABC News' Z. Byron Wolf and Eloise Harper contributed to this report.