US stocks were mixed as markets opened Tuesday after US President Barack Obama and his Republican foes clashed again over the debt ceiling, dimming hopes of a timely resolution to the standoff. The Dow Jones Industrial Average was down 54.46 points (0.43 percent) at 12,538.34 after the first five minutes of trading. The broader S&P 500 slipped 2.25 points (0.17 percent) to 1,335.18, while the tech-heavy Nasdaq Composite climbed 1.40 points (0.05 percent) to stand at 2,844.20. Late on Monday, Obama warned of a \"deep economic crisis\" if Congress failed to raise the US government\'s $14.29 debt ceiling by August 2, while Republicans continued to reject his demand to raise taxes.