OTTAWA — The advice from a top American public relations firm was simple: A Canadian pipeline company should take aim at its opposition.

In detailed proposals submitted in May and August, the public relations firm Edelman outlined a plan to investigate groups that had opposed Energy East, a pipeline in development by TransCanada. Edelman urged TransCanada to develop its own sympathetic supporters and spread any unflattering findings about the opposition.

“We cannot allow opponents to have a free pass,” Edelman advised TransCanada, according to five documents that were obtained by Greenpeace, the environmental group. “To make an informed decision on this project, Canadians need to have a true picture of the motivations not only of the project proponents, but of its opponents as well.”

It is a critical time for TransCanada. The United States Senate is set to vote Tuesday on Keystone XL, the company’s other major pipeline project, which would link Alberta’s oil sands with the Gulf Coast of the United States. Even if the measure passes, President Obama is expected to veto it, citing the environmental review process. Environmentalists have opposed the pipeline, saying energy from the oil sands generates a significant amount of greenhouse gases.