MAUMEE, Ohio—Kicking off his first formal campaign bus tour of 2012, President Barack Obama told several hundred cheering supporters that he would work with Congress to tinker with his signature health care overhaul but stressed that "the law I passed is here to stay."

Mitt Romney has vowed to roll back the Affordable Care Act on "Day One" if he wins in November, a promise that has fired up the Republican Party's conservative base.

Obama, launching a two-day trek through politically pivotal Ohio and Pennsylvania, delivered a ringing defense of his most sweeping domestic policy achievement and said that he was open to improving the law—and working with lawmakers regardless of party to do so."But the law I passed is here to stay," he said in the sweltering heat of the midday sun. "It is going to make the vast majority of Americans more secure."

Obama said the law extended coverage to those who previously lacked it and protected Americans from the predatory practices of insurance companies.

"That's what we fought for, that's what we're going to keep," he declared.