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.