Michael Barone has an excellent analysis here of why Democrats swiftly get into trouble when they go radically left once in power.
There are more conservatives than Republicans and more Democrats than liberals. That's one of the asymmetries between the parties that helps to explain the particular political spot we're in. The numbers are fairly clear. In the 2008 exit poll, 34 percent of voters described themselves as conservatives and 32 percent as Republicans; 39 percent described themselves as Democrats but only 22 percent as liberals.
It's been this way for a long time.
The result is that the two parties have offsetting political advantages. Democrats tend to win on party identification. Republicans tend to win on ideology. Democrats don't have to appeal to as many independents as Republicans do. Republicans don't have to appeal to as many moderates as Democrats do.
Republicans win when they appeal to conservatives. Democrats win when they appear to be moderate while Republicans are busy turning their backs on conservatives.