Inside the sleek hillside headquarters of Valley Health Systems, built with a grant from the health care law, two employees played an advertisement they had helped produce to promote the law’s insurance coverage for young, working-class West Virginians. Health professionals, state officials, social workers, insurance agents and others trying to make the law work for uninsured Americans say the partisan divisions and attack ads have depressed participation in some places. They say the law has been stigmatized for many who could benefit from it, especially in conservative states like West Virginia that have the poorest, most medically underserved populations but...