By deciding to keep a loyal former White House aide out of jail, George W. Bush has done an even bigger favor for his vice president, probably ensuring that he’ll be legally in the clear at the end of his time in office.

The president’s legal finagling last week — commuting the 30-month federal prison sentence of I. Lewis “Scooter” Libby but not pardoning him for the crimes he was convicted of this spring — means that Dick Cheney’s former chief of staff can legally continue to keep silent about his former boss’s role in the events that led to Libby’s lying to authorities and otherwise obstructing justice.

A full pardon would have made it tough, if not impossible, for Libby to claim a Fifth Amendment privilege against self-incrimination if called to testify in a related case — or possibly even before a congressional committee. Since an 1896 Supreme Court ruling, once a convict is pardoned or otherwise given immunity, the constitutional privilege of refusing to give self-incriminating testimony no longer applies.

The legal principle is that whenever a pardon forbids the prosecution of any crimes related to the offense that’s actually being forgiven — which is the boilerplate language in most presidential reprieves — then there is no danger in making self-incriminating statements, and so the shield against doing so becomes simply unnecessary.

In other words, a pardoned convict can be compelled to testify about crimes that he, or anyone else, committed. And so for the White House, a pardoned Libby could be a dangerous thing indeed. Even the casual observer of the Libby trial could readily conclude that the vice president was an active player in the White House leak of CIA operative Valerie Plame’s identity soon after her husband, former Ambassador Joseph C. Wilson IV, sought to rebut a central part of the administration’s rationale for the Iraq War.

The facts and testimony of this case more than suggest that Libby lied to protect his boss, the vice president. So far, Cheney has kept quiet about his role and avoided responding to some troubling questions: Why did he tell Libby, his top aide, that Plame was married to Wilson, who was then bedeviling administration claims of uranium sales to Iraqi dictator Saddam Hussein? What did Cheney want Libby to do with that information?

It does not require stretching the imagination to conclude that Libby outed Plame at his boss’s behest. Once hearing the news from Cheney, Libby passed it on to reporters and other government officials. Is that what Cheney wanted? Getting to the truth is difficult, considering that the vice president isn’t talking and Libby doesn’t have to talk so long as the Fifth Amendment protects him.

The White House has reason to make sure Libby may continue to avoid talking. If Cheney was directing his aide, then the vice president might arguably be involved in the illegal act of unmasking a covert agent.

That could explain why Libby was willing to lie to investigators. He initially asserted that he hadn’t learned about Plame from Cheney. When that became unsustainable by the facts, he changed his story and said he’d forgotten about their conversation. In their opening arguments to the trial jury, Libby’s attorneys signaled that they would use Cheney’s involvement as a defense and even hinted that the vice president could be called to testify as a hostile witness.

Oddly, that much-hyped “Cheney defense” simply disappeared as Libby’s team presented its case. To many, a fair implication to draw was that some sort of arrangement had been made to protect the vice president in exchange for somehow keeping Libby out of jail.

Despite the defense’s capitulation, the trial brought to light just how much the White House might want to keep the country in the dark about what was going on in Cheney’s shop. The testimony revealed how the vice president was personally vested in discrediting Wilson, who was effectively disputing administration claims that Saddam bought nuclear bomb-making materials. Cheney had every reason to tamp down the notion that he might have knowingly let the president repeat a bald inaccuracy as part of the case for going to war. If Cheney had a hand in possibly deceiving Congress and the public in the runup to the war, Libby was in a position to know.

Clearly, a world where Libby tells all he knows is a world that potentially threatens the White House. Allowing him go to prison might have given him an incentive to come clean with prosecutors in hope of a reduced sentence. Hence, the commutation. But a full pardon would no longer protect Libby from being forced to talk. While it seems unlikely that any other prosecutions will arise that could require his testimony at a criminal trial, without the Fifth Amendment privilege he might be compelled to testify to Congress, or in the civil suit the Wilsons have brought against Cheney.

By not pardoning Libby and keeping him out of jail, the president has pardoned Cheney from possibly ever having to explain his role in this bizarre case.

Contributing Editor Craig Crawford is a news analyst for NBC, MSNBC and CNBC. He can be reached at ccrawford@cq.com.