Celebrity talk show host Oprah Winfrey has said she will support Barack Obama for the leadership of the Democrats.
The influential host addressed a rally of around 20,000 people in Iowa urging them to vote for the African-American senator in primaries to be held in the state. The state will be the first to select who will be nominated as the presidential candidate for the Republicans and Democrats.
Ms Winfrey said: "I'm not here to tell you what to think, I am here to ask you to think seriously about the man who knows who we are and knows who we can be. We need Barack Obama," she added.
She praised Mr Obama for his opposition to the war in Iraq and rejected fears that he lacked the experience to deal with the politics on Capitol Hill.
Hillary Clinton currently leads nationwide polls with Mr Obama holding a narrow advantage in Iowa. The primaries in the state, which is the first to cast its vote, are seen as being a key indicator of results in other areas.
The wife of former president Bill Clinton is also holding rallies and visiting parts of the state ahead of the crucial vote.
Her husband has been campaigning on her behalf and Mrs Clinton's mother and daughter Chelsea are expected to make appearances at her rallies soon.