I found this at Ezra's place and it shows that we're getting more liberal. It's interesting how reliably this tracks the prevailing course of domestic policy. During the Eisenhower years, support for liberalism gradually rose. Then it fairly steadily fell throughout the 60s and 70s as the domestic policy climate kept moving to the left. Naturally, with support for liberalism incredibly low around 1980, Ronald Reagan was able to sweep into office. His conservative tendencies rebuilt support for liberalism, which then didn't change very much during the 1988-2000 period when most all big proposals died of gridlock, and the Bush years have led to a massive increase in public support for liberalism.
Of course, to make any sense of this chart you have to understand it as measuring a relative quantity. It's clearly not the case that voters were "more conservative" in 1984 than in 1964 in the sense that 1984 voters wanted to return to the 1964 policy status quo of no Medicare, no Medicaid, no EPA, no Voting Rights Act, no federal funding of education, etc.
Of course, maybe the methodology's all wrong. The chart's put together by Professor James Stimson at UNC and I haven't actually gone back and checked his data or his methods, so caveat emptor.
Matthew Yglesias is a former writer and editor at The Atlantic.