What did workers find was necessary in order to gain leverage in negotiating with employers?

The creation of trade unions.

Explanation:

A trade union is an organization of workers that unite in order to better defend their rights and bargain with their employers. In the early days of capitalism, labor conditions for workers were extremely poor: they worked long hours, earned low wages, and had no labor rights. Workers who complained or try to get better conditions were more often than not fired and replaced, when not subject of reprisals. Workers came to understand that acting indepently they had no leverage, and thus working conditions wouldn't change unless they acted together as a single front. Following the motto "united we stand, divided we fall", workers organized into labor unions, which act as representatives to workers in negotiations with their employers. Unions grew in size, and now range from single workplace unions, to national and even international trade unions.