Barack Obama has warned Americans they face a great challenge, as he prepares to become President in three days’ time.He will take office during the worst financial period in years, with US forces fighting two wars, and with America’s claims of world leadership severely damaged. Obama began his trip to Washington from Philadelphia, a cradle of US democracy. He will follow the route taken by President Abraham Lincoln nearly 150 years ago. “Let all of us do our part to rebuild this country,” he said. “Let us make sure this election is not the end of what we do to change America, but just the beginning. Join me in this effort, join one another in this effort, and together, mindful of our proud history and hopeful for the future let us seek a better world in our time. Thank you Philadelphia!” Obama has stressed it will take time to turn America around, casting a shadow over celebrations for his inauguration.