It depends on the details. If they were more deeply involved in the DNC hacks, perhaps helping plan it in some way, they could be charged with conspiracy for the hacking. That's the least likely. If they knew these leaks were a part of an attempt by the Russian government to influence the election, and they participated and helped with the planning (like timing the releases for maximum impact), they could be charged with conspiracy to interfere in the election. If they coordinated with the Trump campaign during the release of this information, the Trump campaign official(s) involved would be charged with accepting something of value from a foreign national for a campaign, I don't know if that charge would just be against the campaign official or Assange as well.

These are just possible charges, of course. Depending on what Assange knew and when he could have done nothing illegal.