That success happened for one major reason: Democrats made inroads in geographies and constituencies that had trended Republican since the 1960s. In these two elections, a majority of independents and a sizable number of moderate Republicans joined the traditional Democratic base to sweep Democrats to commanding majorities in Congress and to bring Barack Obama to the White House.
Actually the Democrats won the House and the Senate in 2006 because a good number of fiscal conservatives sat on their hands during the election due to the profligate spending habits of the Republican Congress. Independents were upset about both spending and ethics.
The election of Obama in 2008 was largely because the propaganda war that the Democrats had waged on George W. Bush had taken its toll. With their allies in both the print and TV media they had made good progress in painting everything about the Bush administration as corrupt. This in spite of the fact that there were much fewer ethics problems in the Bush administration than there were in the previous Clinton administration.
Obama won the Presidency and the Nobel Peace Prize primarily for not being George W. Bush.
The Democrats will continue on their journey to the left. Any success they have, they have primarily because they have been blessed with the Republicans as opponents.
Since Third Parties are the best way to elect your opponents, what needs to happen is a take over of the Republicans by the “Tea Party” movement. The Democrats will continue their assault but their snarky “tea bagger” comments do not go over well with most Conservative or Independent voters and may cost them votes. Then the “tea party” movement will have to elect Republicans and keep them aware of who it is that elected them.