The Obama administration today proposed overhauling the corporate tax code and lowering the top corporate income tax rate from 35 percent to 28 percent as long as the effort is paid for by slashing tax credits and closing loopholes -- including many of the oil and gas industry breaks that the White House has targeted in the past. The proposal -- which would also set the top tax rate for domestic manufacturers at 25 percent and preserve tax credits for research and development and clean energy -- would be the first major reform of the corporate tax code in decades.