Sen. Clinton and Sen. Obama, you both have battled so far to what is more or less still a draw. Recent history has shown that neither campaign benefits from out front barbs against each other, and while I expect for the "under the radar" campaign to always be back and forth, I would suggest for your public statements to now focus yourselves on one named target: John McCain.
Barring the resurrection of Zombie Regan, McCain is the GOP frontrunner and almost a lock for the nomination. Regardless of the GOP base's disaffection for him, they are still likely to rally around him in order to hold on to the White House. He also has more appeal to independents and Democrats than is comfortable.
In order to counteract that and also demonstrate to primary voters your national appeal, it's time to brutally compare yourselves and the Democratic party versus John McCain and his party. Personalize it, define him anew in the public mindset. Compete with each other over who best knocks McCain off his perch.
McCain is a pro-war supply sider who has expressed his desire to prolong the Bush disaster domestically and internationally. He also happens to be a decade to three decades older than both of you.
Contrast, brutalize, and destroy. Then win.