"McCain jokingly wiped his face, pretending to wipe off moss."

Moss can't be wiped off, and it's not — like the metaphorical egg — only on the face.

In fact, the standard term is mossback:

A reactionary; one who furiously resists progress of any kind.

The word is derived from a sea creature so ancient that it has moss or seaweed growing on its back. During the Civil War it was used to describe those who fled to the swamps and forests to evade the draft; in the 1870s the word was given a political meaning as a Northern counterpart to a Southern "Bourbon (i.e., conservative) Democrat."

Emporia Gazette editor William Allen White, in his famed "What's the Matter with Kansas?" editorial in 1896, thundered: "We have an old moss-back Jacksonian who snorts and howls because there is a bathtub in the State House; we are running that old jay for Governor."

Old jay, eh? Was he wacko? Back to the discussion of mossback (which is from the greatly missed William Safire).

Pennsylvania Congressman J.C. Sibley drew a typical word picture in 1900: "Primitive man lived in caverns, clothed himself with skins, and ate his meat raw, sitting on his haunches, and there has never occurred a change for the higher and better forms of life without arousing the hostility of some old mossback, conservative hunker[s], who will prate of those fairer and better days of old, when their grandfathers swung by their tails from the limbs of the trees."

Wacko monkeys.

Will Rogers told defeated Democratic candidate Al Smith in 1929: "taken out from under the influence of a lot of these old Mossbacks, you are a pretty progressive fellow, Al, and with you and this fellow Roosevelt as a kind of nucleus, I think we can, with the help of some Progressive young Democratic governors and senators and congressmen, make this thing into a Party, instead of a Memory."

Make this thing into a Party, instead of a Memory... a good phrase for Republicans to think about today.

The word was relatively dormant during the thirties and early forties, but President Harry Truman gave it new life in the 1948 campaign. In the Far West he charged that the Republican party's domination by "eastern mossbacks" would stifle the economy of the West. Throughout the country, he denounced the Republican chairmen of the Senate and House committees as "a bunch of mossbacks."

One of the lines that best describes a mossback was leveled at House Speaker "Uncle Joe" Cannon: "If he had been present at Creation, he would have voted for Chaos."...

In 1992 President George H.W. Bush complained that he had extended a "hand of friendship" to Congress at the start of his presidency "and these old mossbacks bit it off." Long Island's Newsday reported a year later that "Clinton joined... Richard Nixon in toasting [Strom] Thurmond's inspiring 90-year journey from racist Democratic segregationist to mossback Republican obstructionist."

The Oxford English dictionary has the old Civil War meaning first, then "A slow, rustic, or old-fashioned person; one attached to antiquated ideas; (hence) an extreme conservative; a reactionary. In early use often denoting farmers of the southern and western States of the United States (see quot. 1888)." Examples:

[1850   H. C. Lewis Odd Leaves from Louisiana Swamp Doctor 181   Here you sit, like a knot in a tree, with the moss commencing to grow on your back.]...
1885   Boston (Mass.) Jrnl. 5 Mar. 2/3   Everybody rejoices over the passage of the bill... We say everybody—we except a few intense mossbacks, who were known during the war as copperheads.
1888   America 18 Oct. 16   Mossback..seems to have originated in the swamps of North Carolina, where a particular class of the poor whites were said to have lived among the cypress until the moss had grown on their backs.
1924   J. Buchan Three Hostages i. 10,   I was becoming such a mossback that I had almost stopped reading the papers.