"Like Lincoln, I ask for your support, your prayers, and for us to 'confidently hope that all will yet be well,'" Obama said.
Lincoln also briefly represented Illinois in Congress before being elected president just in time to preside over the bloody Civil War.
Obama's letter focused on less-dire circumstances and instead cited Illinois citizens he had met on the campaign trail and how he had "learned to disagree without being disagreeable."
CNN said Obama's staff would shut down his Senate office before the end of the year while Gov. Rod Blagojevic ponders a replacement to serve out Obama's term.
Among the Democrats under consideration are Rep. Jesse Jackson Jr. and Tammy Duckworth, an Iraq war veteran who is in the governor's administration.