AMNA NAWAZ: The Supreme Court's landmark decision former President Donald Trump's immunity from some legal prosecution has potential to transform the powers of the presidency.

Our Jeffrey Brown takes a deeper look at how the ruling fits with history.

JEFFREY BROWN: How much power for the executive branch?

What kind of legal restraints?

Those are questions that have been debated since the beginning of the country.

But now, by any account, there's been a major new development.

We look at the past and potential future with historian Heather Cox Richardson, a professor at Boston College.

And welcome back to the program.

Let's do start with history.

What do you see when you look at these early debates about presidential power that might help us think about now?

HEATHER COX RICHARDSON, Boston College: Well, I want to be clear that, in fact, there hasn't been much dispute about the power of the president since the founding of the United States of America.

The people who framed the Constitution as well as the people who wrote the Declaration of Independence, were very clear that they did not want a king, that it was important for the chief executive to have guardrails around him at the time, is what they thought, and that those -- that it was imperative that the president always was answerable to the law.

So we had Alexander Hamilton, for example, in Federalist 69 being very clear that the president could be impeached, the president could be convicted of treason or bribery or high crimes or misdemeanors, could be removed from office, and, crucially, would always, as he said, be liable to prosecution and punishment in the ordinary course of law.

They contrasted that with a king.

Now, that really has not been in dispute, as we know, certainly we have got from 1974, when President Richard Nixon stepped down because he had broken laws and received and accepted a pardon from Gerald Ford, which suggested that he recognized that a president could be held liable for crimes.

And we have had in the confirmation hearings of many of the Supreme Court justices who yesterday overturned that central rule of law saying they too believe the president was under the control, should be under the control of the law.

So this is not a question of we have jockeyed with this.

This is a question of, this is a brand-new development that undermines the central American principle that we are all answerable to the law.

No one is above it.

No one is below it.

JEFFREY BROWN: Let me push back a little.

The majority of the court yesterday says it's distinguishing now between official and unofficial acts.

Now, why is that not a reasonable demarcation line?

Why won't courts in the future be able to distinguish between those?

HEATHER COX RICHARDSON: Well, that was an interesting part of the decision, now, because -- because, as they said, that we have never had to explore what an official act is for the presidency.

What they did was they suggested that the people who would have to arbitrate that would be the court itself.

So, in a way, what they have done is they have set themselves up as the people who got to -- get to decide whether or not what a president does is legal or can be can be prosecuted.

But, just to be clear, this has never come up before, in part because presidents have never been unconstrained by fear of criminal prosecution.

Now, that's not to say that we might not have had presidents who crossed over that line, and we could have a great discussion about who they might have been and what they might have done.

But this is the first time anybody has suggested that a president acting within an official capacity can break the law.

And think about what that looks like.

For example, you could say that, as George W. Bush did with his signing statement, that, regardless of what Congress said about torture, he could engage in that.

Now, think about the things that a president could do.

And, in fact, somebody put on social media yesterday, an A.I.

program that could -- that said, say what crime you want to commit, and A.I.

will tell you how you can say it's an official act.

Think of what somebody who is not liable for criminal acts might behave.

JEFFREY BROWN: Well, what do you fear now?

We have a -- former President Trump has a track record, his first administration.

He's spoken of things he wants to do in the future if elected.

What do you fear and why do you think that these constitutional checks and balances that we have had will not hold?

HEATHER COX RICHARDSON: Well, they're gone.

I mean, that's not -- it's not a question -- people are saying this might be a problem in the future.

No, we're in the problem, because the rule of law, law and order underpins our entire system, the idea that everybody should be treated equally in the courts.

The Supreme Court just ripped that up.

So what am I afraid of?

I'm afraid of, first of all, that people don't recognize what a big deal this is.

This isn't an adjustment in the law.

This is a change in our entire constitutional system.

It says that there is one of the three branches of government that cannot be checked by the other two.

And I don't think that people necessarily understand what that means.

And all you have to do is look to any authoritarian country.

Look, for example, right now in Hungary, where Viktor Orban is busily taking control of other countries' companies that are within his country, because he can do that now.

He's not checked by the courts.

Look at Vladimir Putin's Russia, for example, where he can simply throw his people into the maw of a meat grinder in that war because they can't say no.

We have just -- our Supreme Court has just done the same thing.

JEFFREY BROWN: All right, Heather Cox Richardson, thank you very much.

HEATHER COX RICHARDSON: Thanks for having me.