The prophecy that the Mormon Church would step forward and save the Republic:

Will the Constitution be destroyed? No: it will be held inviolate by this people; and, as Joseph Smith said, “The time will come when the destiny of the nation will hang upon a single thread. At that critical juncture, this people will step forth and save it from the threatened destruction.” It will be so. (JD 7:15; quoted by Brigham Young)

I heard the prophet Joseph Smith say if the people rose up and mobbed us and the authorities countenanced it, they would have mobs to their hearts’ content. I heard him say that the time would come when this nation would so far depart from its original purity, its glory, and its love for freedom and its protection of civil rights and religious rights, that the Constitution of our country would hang as it were by a thread. He said, also, that this people, the sons of Zion, would rise up and save the Constitution and bear it off triumphantly. (Snow, Eliza R. 556)

In the month of May 1843. Several miles east of Nauvoo. The Nauvoo Legion was on parade and review. At the close of which Joseph Smith made some remarks upon our condition as a people and upon our future prospects contrasting our present condition with our past trials and persecutions by the hands of our enemies. Also upon the constitution and government of the United States stating that the time would come when the Constitution and Government would hang by a brittle thread and would be ready to fall into other hands but this people the Latter-day Saints will step forth and save it.

General Scott and part of his staff on the American Army was present on the occasion.

I, James Burgess, was present and testify to the above. (WJS 279; from James Burgess Notebook, LDS Church Archives)

(Donald Q. Cannon, Latter-day Prophets and the United States Constitution, 1991)

This event will not be realized because the “salt” by which America AND Christianity could have been salted has lost its “savor”:

13 Verily, verily, I say unto you, I give unto you [Book of Mormon Stewards] to be the salt of the earth; but if the salt shall lose its savor wherewith shall the earth be salted? The salt shall be thenceforth good for nothing but to be cast out and to be trodden under foot of men. (3 Nephi 12)

The Mormon Church will become trodden under the feet of others in fulfillment of that prophecy instead.