In the letter, the president-elect said he's ending one journey to begin another. He also gave "the people of my beloved home a very affectionate thanks."
The Democratic lawmaker recalled his time community organizing on Chicago's South Side, his service in the state Senate and he quoted Abraham Lincoln. He said he wants to take what he learned from the state to use at the White House.
"The challenges we face as a nation are now more numerous and difficult than when I first arrived in Chicago, but I have no doubt that we can meet them," he wrote.
Obama also asked for the support and prayers of his home state as he heads to the White House.
The following is President-elect Barack Obama's Senate resignation letter.
November 16, 2008
BY PRESIDENT-ELECT BARACK OBAMA
Today, I am ending one journey to begin another. After serving the people of Illinois in the United States Senate -- one of the highest honors and privileges of my life -- I am stepping down as senator to prepare for the responsibilities I will assume as our nation's next president. But I will never forget, and will forever be grateful, to the men and women of this great state who made my life in public service possible.
More than two decades ago, I arrived in Illinois as a young man eager to do my part in building a better America. On the South Side of Chicago, I worked with families who had lost jobs and lost hope when the local steel plant closed. It wasn't easy, but we slowly rebuilt those neighborhoods one block at a time, and in the process I received the best education I ever had.
It's an education that led me to organize a voter registration project in Chicago, stand up for the rights of Illinois families as an attorney and eventually run for the Illinois state Senate. It was in Springfield, in the heartland of America, where I saw all that is America converge -- farmers and teachers, businessmen and laborers, all of them with a story to tell, all of them seeking a seat at the table, all of them clamoring to be heard.
It was there that I learned to disagree without being disagreeable; to seek compromise while holding fast to those principles that can never be compromised, and to always assume the best in people instead of the worst. Later, when I made the decision to run for the United States Senate, the core decency and generosity of the American people is exactly what I saw as I traveled across our great state -- from Chicago to Cairo; from Decatur to Quincy.
I still remember the young woman in East St. Louis who had the grades, the drive and the will but not the money to go to college. I remember the young men and women I met at VFW halls across the state who serve our nation bravely in Iraq and Afghanistan. And I will never forget the workers in Galesburg who faced the closing of a plant they had given their lives to, who wondered how they would provide health care to their sick children with no job and little savings.
Stories like these are why I came to Illinois all those years ago, and they will stay with me when I go to the White House in January. The challenges we face as a nation are now more numerous and difficult than when I first arrived in Chicago, but I have no doubt that we can meet them. For throughout my years in Illinois, I have heard hope as often as I have heard heartache. Where I have seen struggle, I have seen great strength. And in a state as broad and diverse in background and belief as any in our nation, I have found a spirit of unity and purpose that can steer us through the most troubled waters.
It was long ago that another son of Illinois left for Washington. A greater man who spoke to a nation far more divided, Abraham Lincoln, said of his home, "To this place, and the kindness of these people, I owe everything." Today, I feel the same, and like Lincoln, I ask for your support, your prayers, and for us to "confidently hope that all will yet be well."
With your help, along with the service and sacrifice of Americans across the nation who are hungry for change and ready to bring it about, I have faith that all will in fact be well. And it is with that faith, and the high hopes I have for the enduring power of the American idea, that I offer the people of my beloved home a very affectionate thanks.