WASHINGTON (NYTIMES) - This year has brought a barrage of emergencies across the country that have required the National Guard - the coronavirus pandemic, hurricanes, wildfires and a wave of street protests. Now those troops are preparing in case they are needed once again, this time for potential violent unrest in the wake of the election.

Communities are bracing for protest regardless of the election's outcome. If demonstrations turn violent and overwhelm local police, governors will almost certainly call out their states' National Guard.

Under federal law, it is the Guard, not active-duty military, that can enforce order on domestic soil. It has already happened dozens of times this year in cities across the country.

States are already on alert for violence. On Monday, Governor Charlie Baker of Massachusetts ordered 1,000 members of the National Guard to be on standby in case of turmoil following Tuesday's election.

And in Oregon, Governor Kate Brown declared a state of emergency for the Portland area, citing concern for potential violence surrounding the election. Under the plans, the Oregon State Police and the Multnomah County sheriff will take over management of public safety in the city, and Brown said she has directed the National Guard to have members trained in crowd control to be on standby.

"We know that there are some people who might use peaceful election night protests to promote violence and property destruction," Brown said Monday (Nov 2). "That behaviour is not acceptable."

Hundreds of National Guard troops already have been called up in non-law enforcement roles, to assist states where the ranks of poll workers have been depleted by the coronavirus pandemic. Those troops are doing cybersecurity and routine election tasks like opening mail-in ballots. As of last week 10 states had activated the National Guard to help with election tasks, and 14 more are expected to activate troops this week.

New Jersey and Wisconsin have both called up hundreds of citizen soldiers and airmen to work the elections. In both states, the troops helping out are wearing civilian clothes so that voters won't be alarmed by seeing camouflage uniforms at the polls.

"It creates a sense of normalcy," said Major Joe Trovato, a spokesman for the Wisconsin National Guard. "We're not trying to alarm anyone; we are just trying to support the election."

But in a year that is anything but normal, with the electorate sharply polarised, the president warning supporters of a stolen election, and gun sales through the roof, states are bracing for violence that may overwhelm local law enforcement and bring uniformed military troops into the streets.

Legal experts say the election may complicate the response because the president has broad discretion to sidestep legal restrictions by declaring an insurrection, which would allow him not only to take control of state National Guard troops, but also to deploy the Army or Marines.

If the president decides unrest rises to the level of insurrection, there is little Congress or the courts can do to stop him, legal experts say.

"The law is so broadly written that the president gets to decide what's an insurrection, and there is not much local authorities or anyone else can do to stop it," said Rachel VanLandingham, a retired Air Force lieutenant colonel who now teaches national security law at Southwestern Law School in Los Angeles.

President Donald Trump has not indicated he will deploy troops after the election. If he did, legal scholars and historians said it would be highly unusual. But just a few months ago, the president signalled he was willing to use the Insurrection Act to send in federal troops amid widespread protests over abusive policing. "If a city or state refuses to take the actions necessary to defend the life and property of their residents, then I will deploy the United States military and quickly solve the problem for them," he said in June.

But at the same time Trump threatened to deploy active-duty troops, Defence Secretary Mark T. Esper and General Mark A. Milley, the chairman of the Joint Chiefs of Staff, argued against such a move to control the wave of protests in American cities, including Washington, at least at that time.

Esper said at a Pentagon news conference that deploying active-duty forces in a domestic law enforcement role "should only be used as a matter of last resort and only in the most urgent and dire of situations."

A federal law known as the Posse Comitatus Act prohibits federal troops from engaging in domestic law enforcement, leaving that role instead to state National Guard forces.

"The Founding Fathers were wary of having an army interfere in the new republic, so they placed the authority for law enforcement with the states," VanLandingham said. Governors long relied on state militias when local authorities were overwhelmed. Those militias in 1903 were organised as the National Guard but still remain under the control of governors unless they are federalised for a national emergency.

Federal troops haven't been used to guard against election violence since the years after the Civil War when the Army was stationed across the South to put down the Ku Klux Klan and protect Black voters, said Joshua Katzenberg, a retired Air Force lieutenant colonel and military judge who teaches military law at the University of New Mexico School of Law.

"Presidents have historically been very reluctant to call out the Army. They've used the authority briefly and responsibly," he said. But, he cautioned, there are almost no checks on the president's power to send in troops. Challenges in the courts are so slow that they are often decided long after an executive order is carried out, he said, and Congress has no standing to challenge what the White House deems an insurrection.

"A president could issue a ridiculous insurrection proclamation or violate the Posse Comitatus Act, and the people would have very little recourse," he said.