"Union" was the campaign theme for Lincoln's presidential campaign of 1864. This enamel-painted stick pin is an 1864 Abraham Lincoln Presidential Campaign Pin, on eBay at an auction price of $300 with a day and a half to go. [February 21, 2011 update: This campaign pin sold for $795.]
see the original eBay listing
This die-cut shell brass pin is in the shape of a Union shield, featuring 34 stars and 13 stripes. The approximate size of the pin is 7/8" x 1". The stickpin on the back is 1-1/2" long. The seller notes that this 1864 Abraham Lincoln, Shield Shaped, Shell Brass Campaign is pictured on page 64 of Hake / Book III and identified as 3147-N.