President Donald Trump’s pardon of Sheriff Joe Arpaio shows that he continues to be the disrupter-in-chief. Not only does this exercise of power rattle the rule of law, but it also could disrupt special counsel Robert Mueller’s investigation into Russian interference with the 2016 election. However, recent reporting suggests that Trump’s efforts to disrupt the Arpaio case prior to the pardon may have crossed a legal line.

The pardon of Arpaio is different from most other cases because of the nature of the underlying conviction. Most often, the president is exercising his executive power over a matter that was brought entirely by that same executive branch—a federal offense referred by a federal investigative agency and charged by the Department of Justice or one of the 94 U.S. Attorney’s offices. In those instances, the president is showing forgiveness in the interest of the public good. And although the work of the executive branch is cast aside, the public readily accepts that the president, as head of the executive branch, gets to make that call.

Here, on the other hand, the criminal contempt case against Arapaio originated within the judicial branch. The case was referred to DOJ for prosecution by a federal district court judge after Arpaio flagrantly violated the court’s injunction for 18 months in a civil case brought by private parties and then publicly bragged about it. By pardoning Arpaio’s deliberate flouting of the court’s order, Trump is undermining the judicial branch and the rule of law.

Based on statements Trump made at his Arizona rally last week, the purpose of the Arpaio pardon appears to be to please Trump’s political base. Other pardons in history have also had more than a hint of political purpose, such as the pardon of Marc Rich by President Clinton or even the pardon of Richard Nixon by President Ford. But this one is different because of its direct assault on the judiciary. More than Trump’s failure to follow the usual process of consulting with the Department of Justice, waiting five years from the date of conviction, or requiring the offender to show remorse, it is this attack on the judiciary as an independent and co-equal branch of government that makes this use of the pardon particularly harmful to our institutions of government.

The pardon also gives Arpaio a pass for his underlying behavior that landed him court: racially profiling Latino drivers in violation of their constitutional rights, detaining undocumented persons who had committed no crimes when he had no legal right to do so, and enforcing federal law outside of his jurisdiction as a county sheriff. Condoning this illegal conduct further undermines the rule of law and the Constitution itself.

Besides this attack on our independent judiciary, the rule of law, and the Constitution, the pardon also may signal trouble for Mueller’s investigation. Media reports of a search warrant executed at the home of former campaign chairman Paul Manafort and grand jury subpoenas directed at the business interests of former national security advisor Michael Flynn suggest that Mueller is using conventional tactics in building an investigation. He appears to be mounting evidence against lower level offenders in hopes of securing their cooperation against more egregious offenders in exchange for a recommendation to a court for leniency in sentencing. This strategy is sometimes referred to as “climbing the ladder,” “using smaller fish to capture bigger fish,” or “working up the chain.”

But by demonstrating that he is willing to use the pardon power to advance his own interests, Trump is signaling to potential co-conspirators like Manafort and Flynn that they need not play ball with Mueller and his team. They don't need to enter into a plea deal with prosecutors to secure leniency because they can get an even better deal from the president in the form of a pardon.

Of course, after a witness is pardoned, he no longer has criminal exposure in that matter, and cannot invoke the Fifth Amendment right against self-incrimination to refuse to testify. For that reason, Mueller could compel witnesses like Manafort and Flynn to testify even if they were convicted and then pardoned.

Such a scenario, however, is far from ideal for a prosecutor. Lacking an incentive to cooperate, the witnesses would likely answer only those questions Mueller already knows to ask rather than volunteer additional information. Cooperators are used for a variety of purposes to help a prosecution case, such as to provide information about new areas of inquiry, to identify other witnesses and perpetrators, to help connect the dots of disparate pieces of evidence, and to serve as a narrator at trial to describe events and documents. Mueller would lose all of these advantages if he were required to compel witnesses to testify following a pardon.

But in the end, an abuse of the pardon power could do Trump in. While the pardon power is absolute, like all powers, it cannot be used for a corrupt purpose, such as to obstruct justice.

Reports that Trump asked Attorney General Jeff Sessions to drop the Arpaio prosecution could be yet another count of obstruction of justice for Mueller to investigate. If Mueller can show that Trump attempted to interfere with a federal prosecution, that would itself be a crime. Proof of a “corrupt” or bad purpose is also essential. Such purpose could be shown if Trump was seeking to reward an early supporter or to please his political base.

Depending on the timing of the alleged request, it could also tie in to the investigation of obstruction of justice involving Trump’s request to FBI Director James Comey to drop the investigation of Flynn. The meetings with Comey occurred on January 20 and February 14. Comey has testified that shortly thereafter he complained to Sessions about Trump’s inappropriate requests. If Trump was advised about proper channels and topics of communication during that time frame, then that fact could be evidence of his knowledge and willfulness to violate the obstruction statute when he made the request to Sessions to stop the Arpaio case.

The Constitution itself has one meaningful check on the pardon power — the power has no limits “except in cases of impeachment.”

Barbara McQuade is a professor from practice at the University of Michigan Law School. She served as United States Attorney for the Eastern District of Michigan and Co-Chair of the Terrorism and National Security Subcommittee of the Attorney General’s Advisory Committee from 2010-2017.