In "Letter From Birmingham Jail" King is very persuasive by attempting to appeal to emotion. In one paragraph he speaks of the idea behind all his protest is love. He shows this in his writing by writing about all the atrocities committed by police and other white people upon black people during the Civil Rights movement. Even though it is a written letter, the reader can sense the emotion behind it. King is obviously angered by the events but does not come off in his letter as accusatory or venomous.
Being that I am a political science major I chose to look at the campaign website of Barack Obama, particularly his stance on the economy. Most of the writing on the site is not so much supported by emotion as it is fact and promise. His economic policy is mostly proposed compromises such as instating better health care and supporting small business. The web site as a whole is very different in ways of persuasion than "Letter From Birmingham Jail" because it lacks emotion. It is intended to inform and persuade a person to vote for Obama because of his stance on certain issues, not because he is emotionally and dramatically calling for change. I will note that Obama can certainly insight some emotion in people through his speeches as he is an excellent orator.