One book I often mention is Douglas B. Craig's After Wilson: The Struggle for the Democratic Party, 1920–1934. Traditionally, discussions of the Democratic party in the 1920's have focused on "ethnocultural" issues (Prohibition, the Klan, etc.); Craig puts more emphasis on the economic divisions--roughly, between those who wanted a Wilson 1916 type appeal to labor and western farmers and the more conservative elements who sought to gain the support of northeastern business interests. Craig identifies all three of the party's presidential nominees in the 1920's--Cox, Davis, and Smith--with the latter camp (he may in fact somewhat exaggerate Smith's conservatism during the 1920's).
In an old post of mine, I summarized Craig's basic thesis:
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... McAdoo in the 1920s was pretty much the leader of the western/southern section of the Democracy, which believed that the party's nominees of that decade--Cox, Davis, and Smith--were too conservative, too tied to northeastern "big business", and of course too "wet" on Prohibition. The 1928 Democratic platform--which went so far as to endorse the protective tariff--and Smith's making the very wealthy ex-Republican John Raskob head of the Democratic National Committee--were particularly upsetting. McAdoo's friend George Fort Milton said that the 1928 Democrats "want[ed] to forget everything Woodrow Wilson ever thought worthwhile." For the McAdoo wing of the party, the model for a Democratic victory should be Wilson's triumphs--especially that of 1916, where he won despite losing almost the entire Northeast. Smith's backers replied that Wilson's victories of 1912 and 1916 were really aberrations, the first caused by the Taft-TR split, the second by the peace issue, and that the last "normal" Democratic victories were by Cleveland, for whom carrying states like New York, New Jersey, and Connecticut was vital.)
The ethnocultural differences between the two wings of the party have been much stressed, and were real, but it is noteworthy that McAdoo was willing to support a Catholic (Thomas J. Walsh of Montana) for president, so long as he was from the West, progressive on economic issues, "dry" on Prohibition, and untied to urban machines.
Smith's easy victory at the 1928 convention would seem to indicate the increasing weakness of the McAdooites, but this ignores a very important factor: many southern and western Democrats may have felt that Smith should get the nomination because Hoover was bound to win anyway. Smith's defeat in the general election would presumably pave the way for a more satisfactory nominee in 1932, which they hoped would be a better year for the Democrats. Of course this ignored the danger of Smith and his friends like Raskob having control of the DNC machinery. By the time the Depression hit, southern and western Democrats were worried that Raskob would try to make Prohibition repeal virtually the sole issue in the 1932 campaign. They tried to counter this by saying that "bread not booze" should be the issue.
So now enter FDR, who becomes talked about as the Democrats' 1932 candidate as soon as he is elected governor of New York in 1928, and especially after his landslide re-election in 1930. At first the McAdooites viewed him as not that much different from Smith--another Tammany-backed, wet, business-oriented Northeasterner. Eventually, the split between FDR and Smith convinced most of them that FDR's progressivism was for real, and at the 1932 convention FDR's strongest support was from the old McAdoo areas and his weakest support was from his own region (the Northeast, where Smith was strong). McAdoo himself, though, would not back FDR until the last minute, very likely because he still hoped to get the presidential nomination himself (though supposedly he was backing Garner and was only interested in running for the Senate from California). He even cooperated with Smith in a stop-FDR drive!...