Many states will tell the federal government by today if they're willing to help carry out a key part of President Barack Obama's health care overhaul. That's after two years of political battles and a Supreme Court case. At stake are new health insurance exchanges, markets for private coverage that open Jan. 1, 2014. Most customers will get government subsidies to help pay premiums. States must decide if they want to build their own exchanges, partner with Washington, or default to the feds. A check by The Associated Press found that 21 states, plus the District of Columbia, want to build their own or go into partnership. Sixteen are still weighing options. And 13 states appear to be leaving it to the feds. The undecided now have until Dec. 14.