Susan brought this up earlier; I’m just picking up on the thread.
The Obama campaign has a Gardeners for Peace and Obama group; you can join on their website.
And there’s a somewhat boring bumper sticker for sale.
The cool dirt-under-the-nails Obama gear, however, would have to be the Organic Farmers for Obama t-shirts. (Will all this stuff be collectible someday?)
And as if you had to ask: there’s no Gardeners for McCain group. Although you can buy a lame t-shirt.