WASHINGTON -- Missouri Sen. Claire McCaskill will endorse Democratic Sen. Barack Obama for president, The Associated Press has learned.

McCaskill, the state's top Democrat, plans to announce her support for the Illinois senator during a conference call Sunday, according to an Obama aide and a McCaskill staffer who spoke on condition of anonymity.

The endorsement is expected to be a major boost for Obama in Missouri, historically a bellwether in presidential contests and one of nearly two dozen states holding primaries or caucuses on Feb. 5.

Support from McCaskill could also help Obama woo female voters in his race against New York Sen. Hillary Rodham Clinton, his chief rival for the Democratic nomination.

McCaskill has praised Obama often and was widely believed to favor the Illinois senator over Clinton. But the Senate freshman had resisted openly supporting a candidate until now, saying she wanted to preserve working relationships with Senate colleagues.

She said last week that she identifies with the desire for change often voiced by Obama supporters.

Her backing caps a slew of endorsements for Obama over the past week, including former Democratic presidential nominee John Kerry and Arizona Gov. Janet Napolitano.

McCaskill plans to make the endorsement official during a 9:30 a.m. CST conference call on Sunday with reporters.

McCaskill, 54, was narrowly elected to the U.S. Senate in 2006. Obama appeared at a McCaskill for Senate rally days before the 2006 election and drew one of the largest crowds of her campaign.

Both Obama and Clinton have set up extensive campaign operations in Missouri. Clinton has collected her own powerful supporters in the state, including former House Majority Leader Dick Gephardt, former Govs. Bob Holden and Warren Hearnes, St. Louis Mayor Francis Slay and Emanuel Cleaver, a congressman from Kansas City.

Obama has the backing of two St. Louis congressmen, William Lacy Clay and Russ Carnahan, along with St. Louis County Executive Charlie Dooley.