(Bloomberg Opinion) -- It’s been widely reported that President Donald Trump is considering granting a batch of pardons, possibly on his last day in office. Some of the people named as likely beneficiaries have not been convicted or even indicted for any crime.

That raises a question: Does the president have the power to issue a preemptive pardon, one that would protect someone from prosecution in the future? Really?

The answer, given by the Supreme Court in 1866, is yes.

In the 1860s, Augustus Hill Garland was a lawyer in Little Rock, Arkansas, who strongly sympathized with the Confederacy. From 1861 until the end of the Civil War, he represented his state in the Confederate Congress.

That exposed Garland to a future treason charge. In July 1865, President Andrew Johnson pardoned him, “for all offences by him committed, arising from participation, direct or implied,” in the rebellion against the U.S., with the proviso that the pardon would “be void and of no effect if the said A. H. Garland shall hereafter at any time acquire any property whatever in slaves, or make use of slave labor.”

The Supreme Court held that the pardon was legitimate. Speaking broadly, it said that the Constitution “intended to, and in fact did, clothe the President with the power to pardon all offences, and thereby to wash away the legal stain and extinguish all the legal consequences of treason — all penalties, all punishments, and everything in the nature of punishment.”

The court made it clear that a president could accomplish these ends even if Garland had not yet been convicted or even charged. In its view, the president’s pardon power “extends to every offence known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken or during their pendency or after conviction and judgment.”

• Further reading: Trump can’t pardon himself

This conclusion fits with presidential practice, both before and after the court’s decision. President Abraham Lincoln gave preemptive pardons. So did President Jimmy Carter, who pardoned many people who evaded the draft during the Vietnam War, including those who had not been charged. Most famously and controversially, President Gerald Ford gave a “full, free and absolute pardon” to his predecessor, President Richard Nixon, for all federal crimes that Nixon “committed or may have committed or taken part in” during the Watergate scandal and the rest of his presidency.

In light of the Garland decision, it is clear that Trump has the authority to grant White House aides and family members legal immunity against federal prosecution. (State prosecutions are another matter; the pardon power applies only to federal offenses.)

But there are two qualifications.

A president cannot pardon people for crimes they have not committed. Pardons are backward-looking. They are not licenses to commit criminal acts in the future.

There is also a strong argument that the pardons must be specific, in the sense that they must identify the crimes for which people are being pardoned. If that’s correct, Trump could not say (for example) that Jared Kushner, his son-in-law, is pardoned for anything and everything that he might have done wrong. Trump would have to name the crimes to which the pardon is meant to attach.

That could be a problem. Human foresight is limited, and if specificity is required, it’s not clear that Trump, with his small group of advisers, could find a way to give friends and family the broad immunity that he seeks.

It’s true that some past presidential pardons, including Ford’s pardon of Nixon, do not look very specific. But a recent analysis from Aaron Rappaport of the University of California Hastings School of Law makes a powerful argument that specificity is indeed mandatory.

The English practice, before the founding of the U.S. Constitution, was not to give general pardons, but to identify the particular crimes that were involved. As Rappaport explains, this practice served essential purposes, including transparency to the public and protection against inadvertent, careless or uninformed pardons.

William Blackstone, the 18th-century legal thinker who was a defining influence on the American founders, was clear: “General words have … a very imperfect effect in pardons. A pardon of all felonies will not pardon a conviction … (for it is presumed the King knew not of those proceedings,) but the conviction … must be particularly mentioned.” Ideas of this sort, voiced by multiple authorities in Britain, provided the background against which the text of the pardon power in the U.S. Constitution was written.

For Trump, the good news is that preemptive pardons are fine. The less-good news is that there’s a real risk that broad pardons, attempting to provide immunity against prosecution for unspecified misdeeds, might not have their intended effect.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Cass R. Sunstein is a Bloomberg Opinion columnist. He is the author of “Too Much Information” and a co-author of “Nudge: Improving Decisions About Health, Wealth and Happiness.”

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