Here's what I found on the LOC's website.
Special Collections in the Library of Congress
Comic Book Collection
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For more than sixty years, the Library of Congress has acquired comic books through copyright deposit. The current collection of 6,000 titles, probably the largest in the United States, contains approximately one hundred thousand pieces and is growing by about two hundred issues each month. Represented in the collection is the entire range of comic-book subject matter: western, science fiction, detective, adventure, war, romance, horror, and humor. Although scattered issues date from the late 1930s, the holdings are most comprehensive from 1950 to the present. Action Comics, Archie, Detective Comics, Tarzan, and Wonder Woman are among those with fairly complete runs. A record of the holdings, by title, is maintained in the Newspaper and Current Periodical Reading Room. Because of the rapid deterioration of the paper and the value of older issues, full access to the comic book collection is restricted to readers engaged in serious research.