Nick Beaudrot graphs Democratic control of the Senate over time, and concludes that it's unlikely circumstances will ever give the Democrats a chance similar to that of the New Deal era. Yglesias responds, in a very smart post, by quoting Weber's famous line that politics is "the strong and slow boring of hard boards."
First, a little more history. It is in fact relatively rare historically to have liberal working majorities in the House and Senate along with a liberal president. Beginning in 1939, at best there have been twelve years (1961-1966, 1977-1980, and 1993-1994) in which liberals could pass what they wanted (and I'd say 1961-1964 were close calls, and then there's the question about whether Carter counts as a liberal). In the other sixty (plus) years, the problem hasn't been Senate and House rules and procedures, or poor tactics, as much as it was too few liberals in Congress and the Presidency.
On the other hand, the same count for conservatives turns up about the same results: the most generous count would give conservatives control in 1953-1958, 1969-1970, 1981-1982, and 2003-2006, which is fourteen years (but I don't know that I'd count 1969-1970 or 2003-2004, and was Ike really a conservative?).
In other words, most of the time, control is contested. And even one side has a clear advantage, it's usually a limited advantage -- really, Congress was really poised to pass very liberal legislation for a Democratic president was in 1965-1966 and 1977-1978, and there hasn't been an equivalent conservative period. Some of that is institutional barriers (committee system, bicameralism, and the filibuster) but a lot of it is just ideological moderation. The 51st most liberal Senator right now is Conrad or Dorgan or Baucus or Lincoln, not Boxer or Sanders or Harkin. That's a big shift from when it was, oh, Voinovich, but it's not as if we're moving from a Senate full of Brownbacks to one full of Browns.
So, I think Beaudrot is correct that liberals can't really count on a much better environment than this (although very small changes in the Senate can have major effects in terms of what's possible to pass) . And Yglesias puts it nicely when he says that "The process of creating change just requires a sort of realism and gritty determination, that neither gets too discouraged when things don’t work out nor too complacent about the fact that things keep not working out." Indeed, liberals will win (and have already won) quite a few battles this year and next, most likely including something very significant in health care. That they can't do everything they want overnight leads to a lot of frustration, but the fundamental situation is just that it's really, really, hard to get things done in the American system.