WASHINGTON, Feb 26 (Reuters) - President Barack Obama will forecast the biggest U.S. deficit since World War Two in a budget on Thursday that urges a costly overhaul of the healthcare system and would spend billions to arrest the economy's freefall. An eye-popping $1.75 trillion deficit for the 2009 fiscal year is projected in Obama's first budget, according to U.S. officials who briefed reporters on the numbers. That is equal to 12.3 percent of U.S. gross domestic product -- the largest share since 1945 when the country ran a shortfall of 21.5 percent of GDP.