Simple.
Buchanan, being the wasted space that he was, instructs the US army confronting the Nauvoo Legion to enter Utah Territory and subjugate the Mormons. This sparks a religious war in the newly drafted "Deseret".
The Governor of Pennsylvania, highly critical of the President's treatment of Utah mormons, and authorizes militias to volunteer to fight in Utah in defense of the rights and priviliges of their fellow Americans. (This last was threatened should the troops actually engage the mormons during the Utah War of 1857-58. Sometimes called the Mormon War, though there was one similar situation in the 1830s as well.)
The Native Americans of the Western US, who distinguished between "whites" and "Mormons" see their own independence threatened and organize various groups in agreement with Mormon leaders to jointly fight the US military.
As things in the West heat up, other people disheartened by Buchanan (or otherwise contemptuous) begin to speak out and more militias around the country are called up... a growing sense of isolation by Oregon country inhabitants...California sees itself as a potential powerhouse...Mexico, seeing the dissolution of the American West, voids the Treaty of Guadeloupe Hidalgo and reclaims large Southwest territories (possibly offers to aid Deseret and California in exchange for what would be come New Mexico and Arizona)...Texan rumblings?
And so on and so forth. East vs. West