In a video posted on the Internet on Tuesday, McKinney criticized the war in Iraq and complained about Democrats and Republicans, saying both parties are beholden to corrupt corporate interests. She called the Green Party "my new political home."
McKinney, 52, registered to vote in California after a group called Run! Cynthia! Run! began drafting her as the Green Party's candidate there. Since then, she had made campaign appearances in Texas, Oklahoma, Iowa, Illinois, Minnesota and Wisconsin.
In the more than seven-minute video first posted on her Web site on Sunday, then on YouTube on Tuesday, McKinney had particularly harsh words for her former party.
"The Democrats do not speak for us," she said. "The Democrats are no different than their Republican counterparts."
McKinney served six terms representing a suburban Atlanta district but was defeated in 2006 by DeKalb County Commissioner Hank Johnson. She had been the first black woman elected to Congress from Georgia.
One of her final acts in Congress was to introduce a bill to impeach President Bush, saying he misled Congress into approving the war in Iraq and violated the law by secretly spying on citizens. She once claimed Mr. Bush had been warned of the terrorist attacks of Sept. 11, 2001.
McKinney is one of at least seven Green Party candidates for president. The party will select its presidential nominee at its July 10 convention in Chicago.