President Donald Trump's threat to use military force nationwide if governors fail to deploy the National Guard in sufficient numbers to quash protesters is based on the Insurrection Act of 1807.

It is not completely clear whether that can be done without a request from a governor, according to two political experts.

"The U.S. Constitution and the 1807 Insurrection Act and the 1878 Posse Comitatus Act indicate that presidents can send in U.S. troops to stop civil insurrections when requested by states," said Robert Speel, associate professor of political science, Penn State, The Behrend College on Monday night. "But it limits the powers of presidents to do so without state approval."

The law appears to say that presidents can order the U.S. military into action if states are unable to act, which is not true in current circumstances, Speel said. Presidents can also act when states are unable or unwilling to uphold civil rights — such as Southern segregation in the 1950s and 1960s — or if the insurrection is interfering with the execution of federal laws or federal justice.

The Insurrection act can be found here:

The act was last used during the Los Angeles riots in 1992 following the Rodney King verdict, said Nick Clark, associate professor of political science, Susquehanna University in Selinsgrove, Pennsylvania. "However, in that instance, the Act was invoked at the request of then-governor Pete Wilson. The Act is generally reserved for instances in which a state government asks the federal government to intervene."

The president can decide to invoke the act without a gubernatorial invitation, Clark noted, if federal laws are being violated or civil rights are being impeded.

"So there is a legal bar that must be met," he said. "I am guessing the White House would be able to do so and that the Supreme Court would back them up."

Clark believes very few governors are going to support President Trump's approach.

"As of now, no governors have asked for this," he said. "Even Republican governors are going to oppose this in some states, such as Massachusetts and Maryland. You might have Republican governors in more conservative states that request support, like Nebraska, but those will be few and far between."

This will be a partisan issue, Clark said.

"Trump's supporters will be behind him 100 percent," he said. "Liberals and liberal-leaning independents will oppose 100 percent. Conservative-leaning independents and Never-Trump Republicans will likely break in favor of it. Many of them seem to be acknowledging the injustice at the core of the protests, but think that the looting and rioting have gone too far."

Normally, we would expect to see a presidential address by now, Clark said. "That is unlikely to come. Many of his own advisors seem to be against it even though some GOP senators are calling for it. There are policy responses. Justin Amash, the very conservative representative from Michigan, has advanced a proposal to restrict legal immunity for police. His proposal has been supported by opponents of police brutality for years. I doubt the White House gets behind any legislative action. Maybe he will appoint a task force. I believe he will see the use of troops as the defining policy response.

"I don't think it will go over well," Clark said. "I don't think a military response is going to quell this. In fact, it could incite it to even greater levels. Just the threat of it will likely draw more people out of their homes and into the streets."

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