A person from Maryland was called a “Clam Thumper” or a “Craw Thumper in the 1800s. “Craw-thumper” was old slang for a Roman Catholic, and while this also applies to many from Maryland, this is probably not the source of the nickname. Maryland is known for its seafood, and the “craw” or “claw” is that of a lobster. The “thumper” part either refers to fishing or to “thumping” the seafood to crack it open to eat it. “Maryland, Craw-thumpers” was cited in print in 1845.
 
“Clam Thumper” was much less frequently used and has been cited in print since 1864. New Jersey had a similar 19th century nickname in “Clam-catcher.”
 
“Clam Thumoer” is not used today, but “Craw Thumper” is still occasionally used.
     
 
Google Books
April 1845, Cincinnati Miscellany (Cincinnati, OH), pg. 240, col. 1:
Maryland, Craw-thumpers.
 
Chronicling America
23 August 1845, Ripley (MS) Advertiser, pg. 1, cols. 4-5:
NATIONAL NICKNAMES.—It will be seen by the following from an exchange paper that the people of every state have nicknames, and some very curious and ludicrous ones:
 
The inhabitants of Maine, are called Foxes; New Hampshire, Granite Boys; Massachusetts, Bay Staters; Vermont, Green Mountain Boys; Rhode Island, Gun Flints; Connecticut, Wooden Nutmegs; New York, Knickerbockers; New Jersey, Clamcatchers; Pennsylvania, Leatherheads; Delaware, Muskrats; Maryland, Craw-Thumpers; Virginia, Beagles; North Carolina, Weasels; Georgia, Buzzards; Louisiana, Creowls; Alabama, Lizzards; Kentucky, Corn crackers; Tennessee, Cottonmanics; Ohio, Buckeyes; Indiana, Hoosiers; Illinois, Suckers; Missouri, Pewks; Mississippi, Tadpoles; Arkansas, Gophers; Michigan, Wolverines; Florida, Fly-up-the-Creeks; Wisconsin, Badgers; Iowa, Hawkeyes; N. W. Territory, Prairie Dogs; Oregon, Hard Cases.
 
25 July 1864, Indianapolis (IN) Daily Journal, “National Nick-Names,” pg. 4, col. 2:
Mary;and…Clam Thumpers
 
1 December 1865, The Rescue (Sacramento, CA), “National Nicknames,” pg. 3, col. 3:
... Maryland, Clam Thumpers; ...
 
22 March 1866, Louisville (KY) Daily Journal, “Nicknames,” pg. 1, col. 4:
The natives of these States are:
... Maryland, clam-humpers: ...
 
Chronicling America
30 May 1873, The Andrews County Republican (Savannah, MO), “Nicknames,” pg. 3, col. 6:
Maryland—Clam Thumpers.
 
Google Books
U. S.
An Index to the United States of America
Compiled by Malcolm Townsend
Boston, MA: D. Lothrop Company
1890
Pg. 76:
NICKNAMES APPLIED TO THE PEOPLE OF THE STATES.
(...)
Maryland…Craw-thumpers...Lobsters so called by the fishers—craw a corruption of claw, thumper, a longshore localism meaning to bang—the banging or slamming motion of the lobster.
 
Chronicling America
31 March 1891, The Hawaiian Gazette (Honolulu, HI), “Uncle Sam’s Nicknames,” pg. 1, col. 6:
“Clam Thumpers” live in the “Old-Line State,” Maryland, “My Maryland,” and Increase and Multiply” is the watchword of this commonwealth.
 
3 June 1896, The Morning Herald (Lexington, KY), “The Nicknames of Twenty-Eight States of the Union,” pg. 6, col. 3:
Marylanders are called “Craw-thumpers,” a slang name for a lobster.
 
Google Books
Universal Dictionary of the English Language
Edited by Robert Hunter and Charles Morris
New York, NY: Peter Fenelon Collier, Publisher
1897
Pg. 5344:
Maryland. Craw-thumpers (a fisherman’s name for lobsters).
 
Google Books
The World and it People:
Or, a Comprehensive Tour of All Lands
Volume 7
Edited by Charles Francis Horne
New York, NY: Ira R. Hiller
1934
Pg. 2410:
A jesting name by which its people (Maryland—ed.) sometimes call themselves is “craw-thumpers,” a local name for lobsters.
 
23 August 1935, Washington (DC) Post, “Strictly Speaking,” pg. 14, col. 6:
What is the nickname applied to natives of Maryland?
Clam-thumpers.
 
Google Books
Encyclopedia of Maryland
By Nancy Capace
St. Clair Shores, MI: Somerset Publishers, Inc.
1999
Pg. 3:
Maryalnd has two sobriquets, Craw-thumpers and Oysters. The fishermen of Maryland probably first used the expression Craw-thumpers as a nickname for lobsters, after which the fishermen themselves, and later the inhabitants of Maryland, were also designated as Craw-thumpers. This term is composed of the words craw, a corruption of the word claw, and thumper, signifying in the dialect of the local fisherman, a “banger” or a “slammer.” The whole phrase refers to the thumping, banging noise made by the pincers of the lobsters, or by the falling around of the lobsters themselves, (Pg. 6—ed.) as they crawl about in a boat or other receptacle.