Dr. Stockmann becomes the eponymous enemy of the people for speaking out about the pollution of the town's spa waters. Instead of supporting the good doctor, the mayor and local business interests are profoundly hostile towards him, seeing him as a serious threat to the town's prosperity. They know that if it gets out that the spa's water supply is contaminated with deadly bacteria, the town will witness a massive drop in tourism—something it cannot afford.
It would seem, then, that Stockmann, as well as being a responsible citizen for trying to raise the alarm, has a much closer relationship to nature than the mayor or any of the local politicians and businessmen trying so hard to shut him up.
The main bone of contention between the two sides concerns their respective views on the environment. For its part, the political and business establishment of the town clearly see the environment, including the spa's water supply, as an object of exploitation, something to make money from.
Needless to say, Stockmann doesn't agree. He regards the environment as something that should be cherished in itself. It's therefore morally unacceptable to him that the spa waters should be allowed to be contaminated; not just because of the damaging consequences to people's health, but also because such a precious resource should not be polluted under any circumstances in any case.