WASHINGTON — President-elect Barack Obama announced yesterday that he will resign his Senate seat effective Sunday, leaving Illinois Gov. Rod Blagojevich to choose from among a host of willing successors for the final two years of the Democrat's term.

Obama's decision makes it certain he will be absent as Congress undertakes a new round of action on the financial crisis, including proposed relief packages for Detroit automakers and struggling middle-class homeowners, during a lame-duck session next week.

Jim Manley, spokesman for Senate Majority Leader Harry Reid, said Reid was prepared for Obama's resignation, having learned last week that the president-elect would not participate in the upcoming session.

But Manley said Reid's office had no sense of Vice President-elect Joe Biden's plans for departing from the Senate. The Delaware Democrat was re-elected on the same day the Obama-Biden ticket prevailed, and he has held out the possibility of keeping his seat until as late as early January as succession issues are sorted out. Biden's son, Delaware Attorney General Beau Biden, is considered a top contender for appointment to the seat but is serving a tour in Iraq with his National Guard unit.

Obama's resignation could tip the balance of power in the Senate, if only briefly during the lame-duck session, depending on the actions of Sen. Joseph Lieberman of Connecticut. Lieberman, an independent, is a member of the Democratic caucus, but his colleagues remain furious about his relentless campaigning for Sen. John McCain and other Republican candidates this fall.

Democrats will enjoy a dramatically expanded majority when the new Congress is seated in January, and many in the party are eager to strip Lieberman of his chairmanship of the Homeland Security and Governmental Affairs Committee. A vote on Lieberman's fate is expected next week, and if he is ousted as chairman, Senate sources said, Lieberman could cross the aisle to caucus with the GOP.

Manley said Senate officials are researching the implications of such a scenario. But he said that at first glance, there did not appear to be any major organizational repercussions from Obama's resignation, regardless of Lieberman's actions.

The decision to replace Obama rests with Blagojevich, who has appointed a review committee and said he hopes to make a selection by the end of December.

Among the contenders for Obama's seat are two members of Congress from Chicago — Rep. Jesse L. Jackson Jr. and Rep. Jan Schakowsky — and the head of the Illinois Department of Veterans' Affairs, Tammy Duckworth, who is a McKinley High School and University of Hawai'i graduate. All are strong supporters of Obama and have made clear they would jump at the chance to take the appointment.

Other possibilities include Illinois Attorney General Lisa Madigan, a rising political star and potential rival to Blagojevich in his quest for reelection, and Illinois Senate leader Emil Jones Jr., a mentor to Obama in the mid-1990s.