"Those people who would be adversely affected were it to become law are very influential." => [1: Those people [2: who would be adversely affected [3: were it to become law]]] are very influential. => [Those people [who would be adversely affected [if it were to become law]]] are very influential. There are people(1). These people would be adversely affected(2), i.e, they would have negative consequences (take a loss or suffer in some way) in the event that it should become law (3). This is a relative pronoun clause that forms the subject of the main clause - it talks about an uncertain possibility in subjunctive mood. These same people are very influential.