The "base" is typically those towards either extreme, who are very politically active, make contributions, and vote nearly 100% of the time. Those people always vote for the candidate that their party selects, regardless of if that candidate is extreme enough for them.
Nearly ALL liberals think that Clinton is a moderate, and most moderates thing she is a moderate. Warren would be more to most liberals liking (however would lose the moderate vote), but Clinton is close enough to a liberal to get the base vote, plus she will draw in a heck of a lot of moderates, centrists, and those who are just slightly liberal, plus all the women voters who desire to cast a vote for a woman, regardless of lean.
I fully expect a moderate republican candidate as republicans realize that Romney lost the election by pandering to the rich (and assumably far rightwing). Even when he tried to present himself as a moderate during the debates, it was too little too late. Assuming that Clinton get's nominated, both parties will be chasing the moderate vote hard. Moderates are going tho chose sides, and show up in large numbers.