GEOFF BENNETT: President Trump's threat to invoke the Insurrection Act is sparking concern among critics, who warn that the law's sweeping powers make it easy to misuse, especially in moments of domestic unrest.

To help us understand what's at stake, we're joined now by Elizabeth Goitein, senior director of the Liberty and National Security Program at the Brennan Center for Justice.

Thank you for joining us.

ELIZABETH GOITEIN, Brennan Center for Justice: My pleasure.

GEOFF BENNETT: Based on President Trump's past rhetoric and past actions, how seriously should we take the prospect that he might actually invoke the Insurrection Act?

ELIZABETH GOITEIN: I think we need to take it seriously.

He has certainly threatened to invoke the Insurrection Act in the past, but this is coming on the heels of a Supreme Court decision that rejected his efforts to use a different law to deploy the military in U.S.

cities, but that left open the possibility of using the Insurrection Act.

And so I think in the wake of this sort of defeat that he faced in his attempts to deploy the military elsewhere, he might be looking for an opportunity to show that he can still make good on his threats to use the military as a domestic police force inside the United States.

GEOFF BENNETT: And, practically speaking, how would he do it?

ELIZABETH GOITEIN: Well, under the Insurrection Act, he would issue a proclamation.

And then he would have the authority, at least unless the courts held that he didn't, to deploy either active duty troops or to federalize the National Guard and deploy National Guard forces.

And under the Insurrection Act, he can, at least in theory, use them to conduct law enforcement activities.

So the Insurrection Act is generally considered to be an exception to the Posse Comitatus Act, and that's the law that normally prohibits federal armed forces from engaging in civilian law enforcement.

GEOFF BENNETT: How often has the Insurrection Act been used and in what situations?

ELIZABETH GOITEIN: It's been used very sparingly.

And I should say that even though the law on its face is written in vague and archaic language that seems to give the president quite a bit of discretion, the Department of Justice has long interpreted it more narrowly and has opined that it should only be used as a last resort.

And in keeping with that approach, presidents have invoked the Insurrection Act only about 30 times in our nation's history.

And in virtually every one of those instances, either a governor requested assistance because state and local law enforcement were completely overwhelmed, or the state itself was actively obstructing federal civil rights laws.

GEOFF BENNETT: If a president were to deploy federal troops to a state like Minnesota over the objections of the governor and local officials, what would that mean legally?

Where does state authority end federal power begin?

ELIZABETH GOITEIN: Well, the president has the authority to federalize National Guard forces and remove them from state command-and-control when authorized by statute.

And so, if he invoked the Insurrection Act, he would take command-and-control of the National Guard.

Now, the states could file challenges in court and could argue, and I think they would have a strong case, that the president was exceeding his authority under the Insurrection Act and that the actions of the military were in fact violating the Posse Comitatus Act.

In that case, a court could enjoin the president's use of troops.

But in the initial invocation of the act, the president does have the authority to assert command-and-control over federalized National Guard forces.

GEOFF BENNETT: So why, in your view, would this be an illegitimate use of the act?

ELIZABETH GOITEIN: Well, there is chaos in Minneapolis, but it is chaos that is of the federal government's own making.

The vast majority of the violence and lawlessness in Minneapolis is coming from ICE.

And the purpose of invoking the Insurrection Act would not be to quell that violence and lawlessness.

It would be to enable it.

That is clearly an illegitimate use of the Insurrection Act.

And, more than that, it would set an incredibly dangerous precedent.

GEOFF BENNETT: And that plays into what critics argue, that the deployment of these ICE agents to Minneapolis and the protests it sparked, that this was all just a test case to justify invoking the Insurrection Act and potentially deploying the military to Democratic-led cities in this election year, that was the plan all along, that immigration enforcement was really just a pretext, to which you would say what?

ELIZABETH GOITEIN: Well, the president has certainly been very clear that he does not view the use of the military in U.S.

cities as a last resort.

He actually has said that he thinks that U.S.

cities should be training grounds for the U.S.

armed forces.

And if you look at the last nine presidencies combined, not including the first Trump administration, presidents have deployed troops to quell civil unrest or enforce the law only twice.

President Trump has deployed troops or requested or attempted deployment of troops seven times in his first year in office.

So he certainly intends to use the military very differently from the way the presidents have used it in the past and the way that the law has been interpreted to permit the use of the military in the past.

GEOFF BENNETT: Elizabeth Goitein with the Brennan Center for Justice, thanks again for your time this evening.

ELIZABETH GOITEIN: Thanks for having me.