WASHINGTON —  Kash Patel, President-elect Donald Trump’s nominee as the next director of the FBI, has big plans.

He has called for the prosecution of a long list of people he accuses of conspiring to undermine Trump, including President Biden, Vice President Kamala Harris, former Secretary of State Hillary Clinton and outgoing FBI Director Christopher A. Wray.

“These people need to go to prison,” Patel said last year. If he delivers on that threat, he would turn the once-independent FBI into the Federal Bureau of Retribution.

Patel has vowed to purge the federal law enforcement agency of anyone who doesn’t fully support Trump, and says he will transfer all 7,000 employees in the bureau’s Washington headquarters to other cities — apparently including agents who now focus on international terrorism and foreign espionage.

“Go chase down murderers and rapists,” Patel said. “You’re cops. Go be cops.”

On both counts, he is echoing Trump’s long-expressed desire to prosecute his political opponents and bring the FBI to heel.

The president-elect has called on prosecutors to investigate the Biden family, Harris, Clinton, former President Obama, the members of the congressional committee that investigated his attempt to overturn the 2020 election, even the police officers who defended the U.S. Capitol against rioters on Jan. 6, 2021 — “The cops should be charged and the protesters should be freed” — among many others.

And he has long harbored a special animus toward the FBI, which he blames for investigating allegations that his 2016 campaign colluded with Russia and for the 2022 search of his home and social club in Florida that turned up more than 100 classified documents he claimed not to have.

Since his election last month, Trump has said — not entirely reassuringly — that he does not intend to order up investigations from the Oval Office.

“That’s going to be [attorney general nominee] Pam Bondi’s decision, and to a different extent, Kash Patel,” he said last week.

But he added: “If they think that somebody was dishonest or crooked or a corrupt politician, I think he probably has an obligation to do it.”

Patel may not find that a difficult call. He has already published an enemies list of 60 people he considers “corrupt actors of the highest order.”

The record from Trump’s first term suggests that these threats should be taken seriously.

During his four years in the White House, Trump frequently demanded that the FBI and the Justice Department investigate his adversaries. His aides often pushed back, but eventually bowed to his pressure and opened investigations of Clinton, former Secretary of State John F. Kerry, former national security advisor John Bolton, former FBI Director James B. Comey and other former FBI officials. None was charged with a crime.

Those episodes reflect a sobering fact: It’s easier for the FBI to open an investigation than you may think.

“There’s basically no limit, at least when it comes to opening a preliminary investigation,” said Paul Rosenzweig, a former federal prosecutor.

For a full-scale investigation, which could include search warrants and electronic surveillance (if a judge approves), the standards are tougher.

“They have to have an articulable factual basis to believe a federal crime has been committed,” said Michael R. Bromwich, a former Justice Department inspector general. “There’s a lot that can fit within that, but it’s not limitless.”

“If Patel goes to his deputies and says, ‘Let’s open an investigation into Liz Cheney,’ they’re going to ask: ‘What’s the factual predicate?’” he said, referring to the Republican former congresswoman from Wyoming, a vigorous Trump critic. “There will be resistance in the FBI … unless he finds compliant officers who are willing to make something up.”

Prosecution is harder. A criminal indictment requires clear evidence that the person under investigation committed a specific federal crime.

But merely being investigated can have devastating consequences.

“There’s a lot of damage that can be done by an investigation even if there’s no indictment,” Bromwich said. “Investigations are very expensive; a target needs to hire a lawyer. They affect a target’s ability to gain a livelihood. And they are extremely stressful.”

“Lives get ruined,” said Faiza Patel of the Brennan Center for Justice (who is not related to Kash Patel). “People get fired from their jobs.”

An investigation also opens a target’s private life to scrutiny, potentially putting embarrassing information in the hands of the FBI director.

Under J. Edgar Hoover, who ran the FBI for almost half a century until 1972, the bureau assiduously collected private information about politicians and other prominent figures.

The most infamous example was the FBI’s attempt to blackmail civil rights leader Martin Luther King Jr. by threatening to expose his extramarital affairs.

So if a president wants retribution, opening investigations is a good way to start.

The irony, of course, is that Trump and other Republicans have spent years condemning what they claim has been a “weaponization” of the Justice Department and FBI under Democratic presidents.

Now that they’re about to regain the White House, they appear to have decided that weaponization is now their friend.

But senators in both parties should resist that dangerous trend.

They should look carefully at Patel’s skimpy qualifications beyond his loyalty to Trump. In 2020, when Trump proposed giving Patel the No. 2 job in the bureau, his attorney general, William Barr, threatened to quit. “The very idea of moving Patel into a role like this showed a shocking detachment from reality,” Barr wrote later.

They should ask Patel if he realizes that transferring all the FBI’s staff out of Washington would disrupt the bureau’s efforts to stop espionage by Russia and China.

And they should ask whether he really intends to turn the bureau into a weapon of partisan retribution against every target of Trump’s boundless ire.

GOP senators might want to ask why so many of the names on Patel’s enemies list are Republicans who disagreed with him during Trump’s first term, including Barr, Bolton and former Defense Secretary Mark Esper.

Then they should think twice about giving Patel power to investigate anyone he chooses. One day they may find themselves in his sights as well.