I am afraid of them running the White House, Congress and Senate with solid majorities and an ability to steamroll anything they want through the legislative process.
In "A Liberal Supermajority", the Wall Street Journal lays it out better than I can.
If the current polls hold, Barack Obama will win the White House on
November 4 and Democrats will consolidate their Congressional majorities, probably with a filibuster-proof Senate or very close to it. Without the ability to filibuster, the Senate would become like the House, able to pass whatever the majority wants.
Though we doubt most Americans realize it, this would be one of the most profound political and ideological shifts in U.S. history.
Liberals would dominate the entire government in a way they haven't since 1965, or 1933. In other words, the election would mark the restoration of the activist government that fell out of public favor in the 1970s. If the U.S. really is entering a period of unchecked left-wing ascendancy...