WASHINGTON — The Obama administration is expected to release on Wednesday a contentious and long-delayed environmental regulation to reduce ozone, a smog-causing pollutant linked to asthma, heart disease and premature death.

The sweeping regulation, which would aim at smog from power plants and factories across the country, particularly in the Midwest, would be the latest in a series of Environmental Protection Agency controls on air pollution that wafts from smokestacks and tailpipes. Such regulations, released under the authority of the Clean Air Act, have become a hallmark of President Obama’s administration.

Environmentalists and public health advocates have praised the E.P.A. rules as a powerful environmental legacy. Republicans, manufacturers and the fossil fuel industry have sharply criticized them as an example of costly government overreach.

The proposed regulation would lower the current threshold for ozone pollution from 75 parts per billion to a range of 65 to 70 parts per billion, according to people familiar with the plan. That range is less stringent than the standard of 60 parts per billion sought by environmental groups, but the E.P.A. proposal would also seek public comment on a 60 parts-per-billion plan, keeping open the possibility that the final rule could be stricter.