The Democratic Party after the 1984 elections:

[dudeabides]

I would argue that between 1992 and 2004, the party as a whole moved closer to the political center. This was because of Bill Clinton's 1992 presidential campaign - Clinton ran as socially progressive, fiscally centrist. Al Gore tried to portray himself as a moderate during the 2000 campaign, and I'd argue he was successful in that effort.

The 2004 Democratic Presidential Primary is what began moving the party closer to the left. Howard Dean, who had actually been a moderate Governor, brought his anti-war crusade nationally. As a result, the other candidates began moving to the left. In the general election, as expected, John Kerry tried to move back towards the political center - but his record spoke for itself, and President Bush made sure the voters knew Kerry's record. By 2006, the Democrats became as left wing as they had been during the George H.W. Bush administration, and Obama's election as President moved them to the left to an extent not seen since the LBJ era. Obama's efforts to paint himself as a moderate have failed, he won re-election running as a liberal.