They have two angles Mueller appears to be working:
1. Contradicting Obama (official US) policy at the time, such as undermining Russian sanctions while Obama was still President.
2. Obstruction of Justice, by meeting then firing Comey while trying to get Flynn off from being charged for #1 above.
Since no one has has ever been prosecuted for #1 before, it’s obviosly all a a stretch, but since it’s all they seem to have they’re willing to roll with it. They think they’ll have control of Congress by then so it will sail through.
It can’t sail through the Senate where it takes two-thirds to convict.