Mr. Richardson was Energy secretary and United Nations ambassador in the administration of President Clinton. He sought the Democratic presidential nomination this year, but eventually dropped out and endorsed Mr. Obama.
He is one of the nation's most prominent Hispanic politicians, and was in the U.S. House before joining Clinton's Cabinet.
The Democratic officials said Tuesday that Mr. Obama will join Mr. Richardson at a news conference in Chicago on Wednesday. They would speak only on condition of anonymity because the announcement hasn't been made.