It starts with the book of Genesis.
Remember the story about Adam and Eve, and Cain and Abel? (Genesis 4:1-17)
Adam and Eve have sex, Eve gets pregnant, and Cain ("a man from the Lord") is born.
And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD. Genesis 4:1
In the next verse, another son (Abel) is born.
And she again bare his brother Abel. Genesis 4:2a
Cain becomes a farmer, Abel tends sheep, and each offers a sacrifice to God. Cain gave God the fruit of his crops; Abel killed some firstborn sheep and gave God all the fat. God liked the fat more than the fruit. (How could they tell? Did God eat the fat while they watched?)
Abel was a keeper of sheep, but Cain was a tiller of the ground. Cain brought of the fruit of the ground an offering unto the LORD ... Abel ... the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering: But unto Cain and to his offering he had not respect. Genesis 4:2b-5a
That made Cain mad and sad. So he talked things over with his little brother, Abel.
Then he killed him.
And Cain was very wroth, and his countenance fell.
Cain talked with Abel his brother ... and slew him. Genesis 4:4b-8
God dropped by and asked Cain where his brother was. Cain said, "I don't know. Am I my brother's keeper?"
And the LORD said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper? Genesis 4:9
God cursed Cain for killing Abel, making him a fugitive and vagabond.
Now art thou cursed ... a fugitive and a vagabond shalt thou be in the earth. Genesis 4:11-12
Cain told God that he was worried about people killing him, so God put a mark on Cain warning people not to kill him or vengeance would be taken on them seven times. (What were God and Cain worried about? There were only two other people alive at the time -- Cain's parents.)
Cain said unto the LORD ... every one that findeth me shall slay me. And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him. Genesis 4:13-15
Then Cain went off east of Eden in the land of Nod, got married, raised a family, built a city, and did all the things you'd expect a fugitive and vagabond to do on a planet with a total population of 3.
And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch. Genesis 4:16-17
But what was the mark that God put on Cain? Was it a sign on his forehead saying, "I killed my brother, but don't kill me or God will kill you seven times"? The Bible doesn't say.
Lucky for us and Mitt Romney, Mormon Scripture does! (More or less, anyway.)
There are four books in Mormon Scripture: The Bible, the Book of Mormon, the Doctrine and Covenants, and the Pearl of Great Price. And one of the books in the Pearl of Great Price is the Book of Moses.
Here's what Moses had to say about the seed of Cain.
And Enoch also beheld the residue of the people which were the sons of Adam; and they were a mixture of all the seed of Adam save it was the seed of Cain, for the seed of Cain were black, and had not place among them. Moses 7:22
So Mormon scripture says that the seed of Cain are black and God put a mark on Cain. Someone should ask Mitt Romney if the two things are one and the same.
Maybe it should be (Herman) Cain.