According to "The Team of Rivals" by Doris Kearns Goodwin, among the four chief candidates for the Republican nomination in 1860, why did the Republicans choose Lincoln? What is his strongest qualification for the nomination, in the eyes of the Republicans of that day?
As the United States was well on its way to becoming the Ununited States in the late 1850's, the old major political parties (Democrats and Whigs) finally split into various factions, the Southern Whigs joining Democrats, while the Northern Whigs become members of the newly-formed Republicans. However, the Democrats further split along North-South geography, the southern faction nominating John Breckinridge and the northern faction nominating Stephen Douglas. Because the Democratic Party had split and nominated 2 different candidates for the Presidency, it practically assured a Republican victory, no matter who the candidate. The original 8 Republican nominees were culled down to 4 -- New York Governor William Seward, Ohio Governor Salmon Chase, former Missouri Congressman Edward Bates, and former Illinois Congressman Abraham Lincoln.
Ironically, among the Republicans, a split among East-West geography occurred-- Seward, from the East, alienated western supporters because of his pro-immigration views. Among the western candidates, Chase's integrity was called into question from his wheeling and dealing to obtain his Senate seat, and Bates, long in politics, had amassed enough political opposition over the years to deny him the nomination. Lincoln, the least-known politically, caused a minimal amount of alienation among the Republican delegates; additionally, his gift as a story-teller and moderate presentation provided delegates with a level of comfort and security lacking in the other nominees. That confidence enabled them first to work hard for his nomination within the Party; once the nomination was his they then worked hard in the North to insure his election, correctly assessing his chances were poor of carrying any Southern States. Once President, Lincoln assigned Seward as Secretary of State, Chase as Secretary of the Treasury, and Bates as Attorney General.