"In a state that represents the crossroads of a nation, I have met so many men and women who've taken different journeys, but hold common hopes for their children's future. It is these Illinois families and their stories that will stay with me as I leave the United States Senate and begin the hard task of fulfilling the simple hopes and common dreams of all Americans as our nations next president," he said.
Obama must submit his resignation to the Illinois Governor and either the President of the Senate, Vice President Dick Cheney, or one of two other Senate officials.
A Sunday resignation means Obama will not be a senator when and if the chamber convenes next week for a lame-duck session. Obama's Senate Office will remain open for a period of time, but no longer than 60 days after the president-elect's resignation.
The presidential transition office said during that time, Senate staff would work on closing up the office and archiving Senate documents for the presidential library to be built in the future.
The decision to appoint Obama's replacement rests with Illinois Governor Rod Blagojevich.
Those rumored to be under consideration are Rep. Jesse Jackson Jr.; Rep. Luis Gutierrez; Rep. Jan Schakowsky; Tammy Duckworth, head of the Illinois Department of Veterans Affairs; Emil Jones, retiring president of the Illinois state Senate. Blagojevich can also name himself, but has said he's ruled out that option.