This article, which appeared on Slate.com last month, is the most thorough and illuminating exploration of a candidate's position, and character, I've ever read. It explores Mitt Romney's progression from pro-choice Senate and governor candidate to a governor who supported federal funding for stem-cell research to a governor who vetoed any abortion bill whether pro-life or pro-choice to a strongly pro-life presidential candidate. He claimed that he had one view-changing epiphany on the matter in 2004, despite taking positions in 2005 and 2006 which were still pro-choice. He has repeatedly, demonstrably lied about his history on the issue, and the article has all the historical proof to prove it.
It's a very long article, but definitely worth a read. If you're only going to read one thing about a candidate, this should be it.