WASHINGTON — President-elect Barack Obama formally resigned from the Senate yesterday, removing himself from any official role in the lame-duck session of Congress that will convene this week. He also laid more groundwork for the start of his administration by deciding on additional members of his senior staff, including a White House counsel, and preparing to meet with another onetime political rival.

Obama is scheduled to meet today with Sen. John McCain, R-Ariz., whom he defeated in the presidential election, at his transition offices in downtown Chicago. The meeting comes days after the president-elect sat down separately with Sen. Hillary Rodham Clinton, D-N.Y., and New Mexico Gov. Bill Richardson, both of whom opposed him in the Democratic primary.

The president-elect announced his resignation from the Senate in a letter published in Illinois newspapers, telling his constituents, "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."

Saying that he is "ending one journey to begin another," Obama added that he is stepping down "to prepare for the responsibilities I will assume as our nation's next president." Illinois Gov. Rod Blagojevich, a Democrat, has said he will appoint a replacement senator by the end of the year.

The action means Obama will not be part of the congressional debate this week over a stimulus package to jumpstart the nation's struggling economy. He has said the package should include an extension of unemployment benefits and aid for the foundering auto industry, but prospects for passage of that type of plan appear slim.

While Congress debates how best to help the economy, Obama is expected to continue building his governing team. Yesterday, the top tier of his White House staff came further into focus with several new selections.

Washington lawyer Gregory Craig will be White House counsel, according to an individual involved in the transition, and Obama's Senate chief of staff Peter Rouse, 62, was officially announced as a senior White House adviser. Two deputy chiefs of staff were also announced: Jim Messina and Mona Sutphen.

Thus far, Obama's selections have been mostly a mix of Washington veterans — many with ties to the Clinton administration — and trusted campaign aides. Late last week, the president-elect named close friend and adviser Valerie Jarrett as a senior White House aide. Campaign strategist David Axelrod will also hold a senior advisory role.

Obama has not yet announced any Cabinet selections.