Washington, D.C. ? 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 today, 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.