He won't be president until January, but Barack Obama will only be a senator until Sunday.
In a statement, Obama says that's when his resignation will take effect. He was elected to the Illinois Senate seat four years ago.
Under state law, the Democratic governor of Illinois will name Obama's replacement for the remaining two years of his term.
Obama's resignation reduces the Democratic majority to a bare minimum for the post-election session that begins next week. It's a majority that depends, for now, on Independent Sen. Joseph Lieberman of Connecticut siding with Democrats.
Several Democrats want to punish Lieberman for speaking at the Republican National Convention and campaigning for John McCain.
(Copyright 2008 by The Associated Press. All Rights Reserved.)