Attempted witness tampering in a federal felony prosecution is not a matter to be taken lightly. If special counsel Robert Mueller's allegations about this are correct, Paul Manafort has been treating the U.S. District Court and judge presiding over this case, as well as Mueller, the U.S. Department of Justice, and federal criminal law, with the utmost contempt.
Assuming that Mueller can support these allegations with legally admissible and credible evidence (and he has an excellent track record thus far), the court must determine how to deal with this. It will be up to the court's discretion, but in my view, the court should terminate Manafort's home confinement, remand him to a federal prison without bail awaiting trial, confiscate all his communication devices pending trial, and prohibit him from communicating or having contact with anyone pending trial except for his attorneys, spouse and children.
Following the conclusion of the trial, the court should also give consideration to these actions along with any other crimes Manafort is found guilty of in determining what overall sentence of imprisonment would be appropriate.