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.