Dylan Voorhees, a clean energy director for the Natural Resources Council of Maine, said in a statement that Canada's crude oil is a threat to the environment.
"Tar sands oil is very different from conventional oil; it's much, much worse," he said in a statement. "It poses higher safety risks to people and even greater impacts to the environment and climate."
Portland could host a pipeline meant to deliver Canadian crude oil from Montreal. The Portland (Maine) Press Herald described the pipeline as similar to TransCanada's plans to build its Keystone XL pipeline to refineries along the southern U.S. coast.
Critics of tar sands oil point to its corrosive nature and tendency to linger in the environmental if spilled.
Supporters of Keystone XL say those concerns are offset by the employment prospects of building the pipeline and the benefit to U.S. energy security.
"Keystone would have provided a shot in the arm for our nation's uncertain economy, and it could have provided economic opportunity for tens of thousands of families, stretching from here in Texas all the way to the Canadian border," he writes.
Obama rejected the permit because of what he described was an "arbitrary" deadline set by Republicans to approve the project.