An opponent of the Keystone XL pipeline protests outside the Ronald Reagan Building. (Credit: Washington Post)

A group of 150 major Democratic donors and clean energy investors have sent President Obama a letter urging him to deny a presidential permit to the Keystone XL pipeline, comparing the decision's significance to Abraham Lincoln's push to end slavery through a constitutional amendment.

The missive, which was sent by the group Thursday and was obtained by The Washington Post, emphasized Obama's respect for Lincoln and suggested the controversial pipeline--which would transport heavy crude from Alberta to refineries on the Gulf Coast--marked a similar turning point in American history.