Kellyanne Conway, who ran Donald Trump's presidential campaign and now works with his transition team, told MSNBC's Joe Scarborough on Tuesday that the president-elect doesn't plan to follow through with one of the central promises of his campaign.

Conway confirmed Scarborough's reporting that Trump won't “be pursuing investigations" against his Democratic rival, Hillary Clinton, and suggested that Republicans in Congress probably would get that message.

“I think when the president-elect who's also the head of your party now, Joe,” Conway said, “tells you before he's even inaugurated he doesn't wish to pursue these charges, it sends a very strong message, tone and content to the members.

“Look,” she added, “I think he's thinking of many different things as he prepares to become the president of the United States and things that sound like the campaign aren't among them.”

There's a tacit admission there that Trump's insistence that he would prosecute Clinton was all campaign rhetoric. But what's missing from Conway's response is the simple, important fact that Trump, as president, doesn't have the authority to demand that Clinton be prosecuted — and that it's a good thing that no president does.

We spoke by phone with Columbia University professor Daniel Richman, a former federal prosecutor who has consulted with the Justice and Treasury departments. He explained how and where the president can leverage his authority over the government's criminal investigatory mechanisms.

Richman said that the president can, through his attorney general, target broad areas for focus.

“Certainly as the head of the executive branch, the president has considerable sway over policy decisions, as to what kinds of cases or what types of offenses will get priority,” he said, referring to things like corruption or fraud. “But a huge line has always been drawn between general priorities and specific cases, and there are a considerable number of conventions, protections and institutional frameworks in place to keep presidents out of particular cases.”

Richman points out that it's not completely unheard of for a president to seek to target individual people, citing the example of New York drug dealer Nicky Barnes, who appeared on the front of the New York Times Magazine in 1977 to President Jimmy Carter's great annoyance.

“But particularly post-Watergate, there really have been efforts to very much patrol communications between the White House and the Justice Department,” he said. “Usually there is a designated person in the [White House] counsel's office; there are very closely watched and monitored counterparties in the Justice Department to make sure that very little is done with respect to particular cases.”

The goal is simple: to maintain the independence of federal investigators and prosecutors from political influence. “I think the bottom line is, I think both sides realize that if prosecutors become seen as carrying water for the president, it will not be a good thing for federal enforcement in general, or for that case in particular,” he said. The system is designed to prevent the occupant of the White House from using federal investigations for political purposes in precisely the way that Trump once threatened to do.

The Washington Post's Ellen Nakashima spoke with another expert who echoed the same idea.

"Once again, the president-elect has demonstrated his complete lack of understanding of how the government makes these kinds of decisions," said Stephen I. Vladeck, a law professor at the University of Texas at Austin. "The attorney general answers to the president, but the department is supposed to be independent, especially when it comes to prosecutorial decisions. Any president, especially our next president, needs to both understand and respect that – or else they risk politicizing criminal prosecutions in ways that can be damaging."

Giuliani weighs in on prosecuting Clinton over email server (The Washington Post)

Richman pointed out how many layers a potential prosecution would need to filter through: the FBI, the lawyers in the Justice Department (who, he points out, “generally are not shrinking violets when it comes to complaints of political interference"), the judiciary — and ultimately the jury that would have to convict.

“There are all these different institutions that are required for a criminal case to go forward and to conclusion that are not beholden to the president and are very much not intended to be beholden to the president,” he added.

In brushing away the idea that Trump would transform his frequent campaign pledge into reality, Conway suggested that the president-elect was being generous, not that he didn't have the power to act anyway.

“I think Hillary Clinton still has to face the fact that a majority of Americans don't find her honest or trustworthy,” Conway added, “but if Donald Trump can help her heal, then perhaps that's a good thing.”

In a Fox News Channel poll conducted right before the election, a majority of Americans said they thought Trump wasn't honest or trustworthy either.

Ellen Nakashima contributed to this report.