I'll spoil it for anyone who hasn't figured it out. It's in the page source. In the beginning of the page, after the Doctype string and the lines for older versions of Internet Explorer, there is an ASCII art image of the Obama "O" logo.
Edit: And it's done with the letters u, s, a, and just a few kinds of punctuation marks. (Thanks to stephanerangaya.)
Further edit: No obvious easter eggs in the source of the Mitt Romney campaign page. I am disappoint.
They decided recently this is a developer feature, so you have to enable the "Show Develop menu" feature (in the "Advanced" pane of Safari's preferences window).
This irritated me at first, but then I recalled the time my dad (accidentally) viewed a website's source, and how perplexing it was to him.
Okay, I feel better knowing that it's one configuration change away. The Internet had lead me to believe that I needed to download a plugin to make this work. :-/
That said, their fancy source navigation tool breaks the formatting of the ascii art. :-(