You nay-sayers beware, I have a crack team of historians available at my beck and call to provide the intellectual heft to back up my vituperative utterances.
New Deal stud Eric Rauchway (the "anti-Amity Shlaes") of the indispensable "Edge of the American West" explains FDR's 1938 campaign to try and purge conservative southern Democratic senators who opposed his legislative agenda. FDR failed miserably and his defeat signaled the death knell of the New Deal. (Noted liberal ally Hitler helpfully began making additional mischief in Europe around this time thereby giving FDR a new raison d'etre.)
Why is this relevant? [Because I fucking well said so and you don't want me to have to stop this blog, mister!] But in addition to that good and sound reason, it is illustrative of the limits of the power of even the most popular president, in the most difficult of circumstances, in terms of being able to impose party discipline on a sitting senator, particularly in places such as the southern U.S. (or say Nebraska). Those who are calling on Obama to bring the hammer down on the party's mushy middlers are just not being clear-headed about the leverage that Obama actually possesses in places like Nebraska, Arkansas, or Louisiana, i.e. more or less none.
And that's one of the reasons that the original Social Security Act passed under FDR "sold out" huge chunks of the southern, African-American work force by exempting agricultural and domestic workers from its coverage. Even our most vaunted liberal president, blessed with huge majorities in both houses, winner of a crushing landslide victory that gave him a mandate to tackle the Great Depression, still had to count votes.
Again, let's give Oama just a little bit more time before we denounce him as a sell-out and failure.