Mr. Trump’s action does nothing to change the past.

But it might change the lives or convictions of people whom President Trump does know: his own personal firewall. By pardoning Mr. Libby, Mr. Trump sends a message to Paul Manafort, Michael Cohen and any of his other close aides who are facing or may face potential prosecution pursuant to the investigation by Robert Mueller, the special counsel.

Mr. Manafort was indicted in October for hiding that he was working for a Russian-backed Ukrainian party while lobbying in the United States; charges against him could put him away for the rest of his life. F.B.I. agents raided the home and offices of Mr. Trump’s longtime lawyer Michael Cohen this week; according to the Department of Justice, he is under criminal investigation by the Southern District of New York, and he may face charges of bank and wire fraud for paying hush money to prevent news of past sexual affairs from becoming public during the election. By pardoning Mr. Libby, Mr. Trump sends a message to those who might incriminate him in crimes related to conspiring with Russians to tamper with the election: The message is that he will rectify any sadness that protecting a president might cause.

The thing is, Mr. Trump is unlikely to be able to use his pardon power to get out of his legal jam. That’s because several of his potential firewalls — Mr. Manafort, Mr. Cohen and his son-in-law, Jared Kushner — could be charged at the state level for the financial crimes they’re suspected of. A federal pardon would simply move their prosecution beyond Mr. Trump’s control.

And there are many more people who can incriminate the president, whereas in the investigation into Ms. Plame’s exposure, Mr. Libby was one of the only people who could say whether the president had authorized the leak of a C.I.A. officer’s identity. Already, three key witnesses have agreed to cooperate with Mr. Mueller against the president, so it’s probably too late to start silencing witnesses.

Finally, neither Mr. Trump nor his thoroughly outmatched legal team knows the full exposure he or potential witnesses face. Given the involvement of Russians trying to undermine the United States, the evidence Mr. Mueller may already have collected could well be even uglier than deliberately burning a C.I.A. spy for political gain.

That makes it a lot harder to pull off what George Bush did — protect his firewall.

Finally, Mr. Trump is running out of time. As NBC reported this week, Mr. Mueller is already preparing the first of two reports to Congress. This one will lay out the ways the president has already obstructed his investigation into election tampering. It will reportedly include the discussion of pardons with Mr. Manafort and Mr. Trump’s former national security adviser, Mike Flynn (who pleaded guilty and agreed to cooperate in November). In other words, within weeks Mr. Mueller will inform Congress that President Trump has been offering pardons specifically to undercut the investigation into his actions.

Mr. Trump’s pardon of Mr. Libby makes it crystal clear that he thinks even the crime of making the country less safe can be excused if done in the service of protecting the president. But it doesn’t mean the pardon will protect him.