The most memorable letter-as-logo is Barack Obama's "O," but before him, Hubert H. Humphrey (D) used a triple "H" in 1968, and one iteration of George W. Bush's (R) 2004 logo used just the "W." In 2012, Mitt Romney (R) used an "R," and this year, Hillary Clinton (D) is going with an "H," former Maryland governor Martin O'Malley (D) sometimes uses an "O'M" instead of spelling his name out, and Rick Perry (R) has a "P."
The Humphrey and Bush examples were aberrations -- more a product of the novelty of the candidates' initials than part of a larger trend. Single letters as symbols are of course nothing new, stretching back to royal seals and monograms, but today's letter logos share a debt to the Obama "O" as well as smartphone-friendly design trends.
Obama's logo took his last name, foreign-sounding and one letter away from being the same as Public Enemy No. 1, and turned it into a symbol that evoked his hope and change message with a rising sun over a rolling hill. It was unlike anything seen before in presidential campaigns, more hip than your run-of-the-mill stars-and-stripes campaign logo, and also more corporate. Obama was a brand.
We see companies forgo spelling their name out for just a letter or symbol all the time. McDonald's uses just its "M" and Starbucks dropped all letters around its green siren logo. It's especially common with tech companies, where symbols or single letters fit better in the thumbnail confines of square icons than long logos do. Facebook is a lowercase "f," Pinterest is a "P," Tumblr is a lowercase "t," and the Washington Post app, in case you haven't downloaded it yet for some reason, is a "WP."
Single letters are optimized for our smartphones, whether that's for a button on our homescreen or an avatar on social media. We see that with O'Malley, who spells out his last name for his logo on campaign signs, but uses the "O'M" for his Twitter profile. Both, by the way, are set inside what resembles an iMessage text bubble.
Single letters also work well in a crowded market. Just as strong corporate logos help products and their associated brand attributes stand out in our mind and on store shelves, strong campaign logos can quickly help voters recall candidates' message and brand. Clinton's "H" brings to mind moving forward (and/or to the right, not the left, as people have pointed out online), O'Malley's "O'M" is a conversation, Perry's "P" is a "The More You Know" star, and Jindal's "J" ... well, it kind of looks like a remixed Obama "O," cut up, stretched, and reworked in Photoshop.
In a presidential campaign that now has a dozen officially declared candidates and counting, politicians must set themselves and their brand apart from their competition. The logo is one way they do that, and increasingly with as few letters as possible.
This post has been updated to include Rick Perry's "P" and Mitt Romney's "R."