By: Andrew Beaujon | Poynter
The “District” in “District of Columbia” can now be capitalized on second reference, AP said in a Stylebook update in March. (I didn’t notice it until AP reporter Ben Nuckols tweeted about it this morning; this change was overshadowed somewhat by AP’s shift on “over” and “more than,” one of the most important events in Stylebook history.)
District of Columbia Abbreviate as D.C. in datelines or stories. Postal code: DC. Do not use D.C. in conjunction with the federal district of Washington unless it could be confused with other Washingtons. On second reference, the District is acceptable.