The retirement of Byron Dorgan is a solid example of why, in many cases, legislation is at the mercy of conservative-leaning states, and nothing the president can- or should do will change it.
With Dorgan out, it's very likely that a Republican will win that seat. If Ben Nelson were to resign, he'd probably be replaced by a Republican. Blanche Lincoln is likely to be replaced by a Republican. It goes without saying that those Republicans won't be very receptive to robust public options or passing the Obama agenda.
We're more or less stuck.
No amount of presidential leadership or "pushing" from the left will change this. We can't get consistently liberal votes out of these right-leaning midwestern and southern seats until the ideology on the ground is shifted. Grassroots promotion of progressive ideas door-to-door. It's a long process of transformation, but with some patience and hard work, it can be done.
Once we make these states safer for Democrats and progressivism, we'll get more liberal votes in the Senate. In the meantime, expecting the president to yell at people like Ben Nelson in hopes that he'll suddenly forget that he's from Nebraska is just unrealistic.