US President Barack Obama has delivered his state of the Union address with a plea for unity. Focusing primarily on the economy, he said the US must respond to the challenges posed by increasingly competitive world. He was speaking to a divided Congress, with the Republicans now in control of the House. But in a break with tradition, Republicans and Democrats sat together - instead of across the aisle in show of respect for wounded lawmaker Gabrielle Giffords and six people who were killed in Tucson.