Perhaps this is obvious. But it seems to me that the real reason the Democratic primary race has gone from heated to vicious (at least among the candidates' supporters, if not the candidates themselves) is precisely because we're in this awkward seven week hiatus in which there are no actual elections being held. Actual voting, rightly, has served as the closest thing to a referee this on-going contest has. So each side would have it at for a week or so. And then we'd have some voting. And despite all the efforts to spin the results on both sides they'd still have an undeniable effect. After South Carolina, Hillary seriously rejiggered her approach. After Ohio and Texas, Obama's camp decided that certain attacks against them had stuck. All the acrimony and spin notwithstanding, the regular input of voters had the effect of keeping the campaigns on something like a common narrative. Without them, we are stuck with the same, unchanging stubborn set of facts: Obama has a relatively narrow lead which, under the DNC's rules, is nevertheless extremely difficult to overcome. And each side is left cycling over into more and more heated iterations of the same arguments, like a cascade into mounting levels of mania, at least among supporters if not always the campaigns themselves.
That doesn't mean that both side's arguments have equal merit. For my part, I think the Obama campaign has far the better part of it. But I think it does explain why we're now in this self-escalating spiral.