Here's a brief passage from Jean Edward Smith's FDR biography that I wish he'd said more about:
The caucus resolution gave Majority Leader Joseph Robinson authority to convene the caucus “for the purpose of considering any measure recommended by the President and that all Democratic senators shall be bound by vote of the majority of the conference.” The New York Times, March 7, 1933. The prior caucus rule had required a two-thirds vote. In the House of Representatives, it continued to require a two-thirds vote to bind the Democratic caucus.
That seems like it goes a long way toward explaining the difference between the speed with which the first New Deal congress enacted reforms and the relatively sluggish pace of the 111th Senate.
But of course it's pretty obvious why Senate Democrats didn't do any such thing—it's an enormous cession of power away from individual senators and toward the White House. So why were the 1933 Senators so much more willing to be self-abnegating? From a contemporary perspective, the speed of that early New Deal legislation and the willingness of congress to delegate power to the executive is striking.