In a very good post, Tristero essentially asks why can't Democrats speak like other humans? While there's been a lot of good discussion at that post, I think the answer has to do with the way campaigns are funded. Typically, when we think of the skills that a politician needs, the ability to speak clearly would appear to be an important, perhaps the most important skill.
The most important skill for the modern politician is to be able to raise $2,000 dollar donations from wealthy people. While this might not damage one's ability to speak clearly to a much broader audience, there's little or no obvious correlation between the two skills. For Democrats, this weeds out many skilled communicators: my experience has been that most people left of center really don't like asking others for money, particularly if there is even the remote expectation of a quid pro quo in return. Consequently, the Democrats lose many otherwise qualified candidates because of their unwillingness or inability to cold-call for cash.
This doesn't hurt the Republicans nearly as much because the quid pro quo is part and parcel of the modern conservative's 'ethics' (see the entire Bush Administration). Consequently, they lose far fewer of their best communicators.
The need to raise huge sums of money enables someone who is linguistically challenged like Feinstein to be elected and re-elected. She has the one linguistic skill that matters: the ability to talk people into giving her their money.
By contrast, Democrats tend to forget about the Republicans' highly effective direct mail operations. These operations have forced the Republicans to figure out how to speak to a much broader audience. Perhaps the greatest political potential of the internet will be its ability to force Democratic politicians to speak to a much wider audience to raise money. This could force them to communicate more with rank and file Democrats (or at least those Democrats who are not rich enough to drop thousands of dollars on politicians).
This is one more reason why publicly funded campaigns would be a good idea.