They have nothing. For all this investigation Mueller found nothing which is why he shoveled it off to the Grand Jury. Remember Darren Wilson. There was no evidence whatsoever that he lacked justification for shooting Michael Brown so it was shoved off to a Grand Jury to take the blame.
Mueller's last hope is if he can lean on Manafort hard enough to parrot a story that will stand up to scrutiny. Failing that, Mueller can say "See, we didn't waste time, we got someone for something".
Right now, Comey is in the pot and the water is starting to boil. If Comey cooks, he will take Mueller right along with him. Or, Mueller can throw Comey under the bus and claim he was misled all along.