As John Cole puts it:
Grab the popcorn and soda, because the GOP is providing the nuts.
Here is a section from the official LDS site, explaining Mormon theology on the question:
On first hearing, the doctrine that Lucifer and our Lord, Jesus Christ, are brothers may seem surprising to someespecially to those unacquainted with latter-day revelations. But both the scriptures and the prophets affirm that Jesus Christ and Lucifer are indeed offspring of our Heavenly Father and, therefore, spirit brothers. Jesus Christ was with the Father from the beginning. Lucifer, too, was an angel “who was in authority in the presence of God,” a “son of the morning.” (See Isa. 14:12; D&C 76:2527.) Both Jesus and Lucifer were strong leaders with great knowledge and influence. But as the Firstborn of the Father, Jesus was Lucifer’s older brother. (See Col. 1:15; D&C 93:21.)
How could two such great spirits become so totally opposite? The answer lies in the principle of agency, which has existed from all eternity. (See D&C 93:3031.) Of Lucifer, the scripture says that because of rebellion “he became Satan, yea, even the devil, the father of all lies.” (Moses 4:4.) Note that he was not created evil, but became Satan by his own choice.
When our Father in Heaven presented his plan of salvation, Jesus sustained the plan and his part in it, giving the glory to God, to whom it properly belonged. Lucifer, on the other hand, sought power, honor, and glory only for himself. (See Isa. 14:1314; Moses 4:12.) When his modification of the Father’s plan was rejected, he rebelled against God and was subsequently cast out of heaven with those who had sided with him. (See Rev. 12:79; D&C 29:3637.)
I know this is relatively obscure theology, but, hey, this is the GOP. If they insist on fusing religion with politics, the rest of us have to bone up on Mormon teachings.