Can Congress add an amendment preventing a president from pardoning a person who has refused...

... to testify against him?

Is there any reason, other than the GOP Senate, that the amendment couldn’t be at least introduced? This might give Stone et al something to think about. Plus it would simply be fair. Nobody else in the entire country has the ability to persuade people to refuse to cooperate—that is, to tamper with a witness with a carrot instead of a stick—without consequences.

I disagree with the principle of the president’s being unindictable and above the law... but I can see how it’s defensible. Normal presidents don’t have time to be distracted by a legal proceeding against them. But I don’t see why the ability to pardon a coconspirator should be one of the perks of the presidency.