I wouldn't say that. Look at Wyoming's Governor for the last 8 years. He's a Democrat, and one of the most conservative Governors in the U.S. It is possible to be a Dem, and not be an idiot, just as it is possible to be Repub, and still be an asshat.
Only if you don't need the campaign funds.
In fact, it is not possible to support the Democratic Party with out being an idiot because the party platform is based on stupid ideas that have failed miserably every where else they have been tried.
3 quick examples, Stimulus, National health care, more power to the unions.
Each of these along with gun control have been major components of the Dem platform for decades.
So again, No, you can't be a Dem with out being an idiot.