So I've been working on a TL in which a mosquito bite in 1844 leads to an early Civil War and a Confederate victory. I've got about 6400 words so far and am through the Mexican-American War. Here's how it starts.
May 24, 1844: Gideon Pillow, member of the Tennessee delegation to the 1844 Democratic National Convention and close friend of James K. Polk, arrives in Washington, D.C. on his way to Baltimore. A mosquito lands on his hand while he dines with friends, and he hesitates to brush it off, more concerned with finishing his point than the annoying pest. It bites him before he squashes it.
May 27, 1844: The Democratic National Convention opens in Baltimore. Pillow lies in agony 37 miles away in DC, having contracted malaria from a mosquito bite. He recovers, but during the convention his place is taken by Andrew Johnson, an alternate delegate. In regards to the presidential nominee, the convention is intensely divided between supporters of former President Martin van Buren and those of General Lewis Cass. Cass has the support of many westerners, while van Buren has the support of New England. The major issue is the annexation of Texas, then an independent republic at war with Mexico. Van Buren opposes annexation; Cass supports it.
Van Buren’s opposition to the annexation of Texas, as well as his abolitionism, cause him to lose support among westerners and the South. Thanks to a recently changed rule of procedure, candidates must receive two-thirds of the delegates in order to win the nomination. Van Buren’s lack of support in the South and West means he lacks the necessary two-thirds, and numerous run-off ballots are held in order to determine a nominee. Cass gains support throughout this process, until he has the lead in delegates. However, he too lacks the necessary two-thirds.
OTL: George Bancroft, a delegate from Massachusetts, approaches Gideon Pillow about the possibility of running Polk, who had initially planned to contend for van Buren’s vice-presidency, as a compromise candidate. Pillow immediately and enthusiastically seizes on this idea, and on the eighth ballot Polk is introduced as a compromise. He wins with a unanimous vote on the ninth ballot.
ATL: Without Pillow, Andrew Johnson takes over leadership of the Tennessee delegation. No fan of Polk, Johnson is willing to throw him overboard in order to give the nomination to Cass. When Bancroft proposes to Johnson that Polk be put forward as a compromise candidate, Johnson rejects his suggestion out of hand. However, hearing of this, Lucius Lyon, a supporter of Cass, approaches Johnson after the seventh ballot. Lyon proposes that Johnson muster support for Cass among Polk’s people, and in exchange, Cass will choose Polk as his running mate. Johnson’s reply: “I’ll think about it.”
Johnson does, and on the morning of May 29th, he accepts Lyons’ proposal. On the ninth ballot, Cass wins 181 delegates, enough to secure the nomination. He is the Democratic candidate for President in 1844.