William G. Mayer 
The Divided Democrats [PDF ebook] 
Ideological Unity, Party Reform, And Presidential Elections

Do Democrats have greater difficulty maintaining party unity than Republicans? William Mayer argues that they do, chiefly because the Democrats are a more ideologically diverse, less cohesive party. After extensively documenting the Democrats“ traditional problems of division and disagreement, Mayer presents evidence suggesting that the Republican advantage over the Democrats has finally started to narrow—raising important questions about the future of the Republican coalition.