In a coup for House Democrats, AARP will endorse sweeping health care overhaul legislation headed for a history-making floor vote, officials told The Associated Press on Wednesday.

An endorsement from the seniors' lobby was critical when then-President George W. Bush pushed the Medicare prescription drug benefit through a closely divided Congress in 2003. House Democratic leaders are hoping it will work the same political magic for them as they strive to deliver on President Obama's signature issue.

An announcement from the 40-million member group is expected Thursday, said officials with knowledge of the group's decision. They spoke on condition of anonymity because the endorsement is not official yet...