GOV. CHARLIE BAKER filed a court brief on Friday defending his use of a 1950 Civil Defense Law to declare a COVID-19 state of emergency, equating the virus to the “natural causes” referenced in the law and pointing out that the Legislature has not balked at his use of emergency powers to shut down the state’s economy.

The brief, which includes citations to Alexander Hamilton and The Federalist Papers, is the first time Baker has publicly spelled out in any detail his legal authority for declaring a state of emergency.

Attorney General Maura Healey filed the brief on behalf of the governor in response to a lawsuit filed by a group of business owners and pastors who say the Civil Defense Law is not applicable to COVID-19 and the governor’s many sweeping orders to deal with the coronavirus infringe on powers granted to the Legislature under the state constitution. The parties are scheduled to appear before a single justice of the Supreme Judicial Court on September 11.

The Civil Defense Act of 1950 was passed at the beginning of the Cold War, a time when the United States was increasingly worried about the spread of communism and military threats from the Soviet Union. The law allows the governor to declare a state of emergency when the state is threatened by enemy attack, sabotage, riots, fires, floods, earthquakes, droughts, or “other natural causes.”

According to the state website, all but one of the previous state of emergency declarations in Massachusetts dealt with storms. The lone exception was a state of emergency declared by Baker to address the Merrimack Valley gas explosions in 2018.

Michael DeGrandis, an attorney with the New Civil Liberties Alliance in Washington, DC, who filed the original lawsuit, said COVID-19 is a health, not a civil defense, crisis. He said the Public Health Act is the statute that should be used to address COVID-19 because one of its main purposes is to control and prevent the spread of infectious diseases.

In her legal filing, Healey argues that Baker has broad discretion under the Civil Defense Act to determine whether a disaster arises from “other natural causes.”

The brief says the coronavirus qualifies because it has “infected millions of people, overwhelmed public health systems, and killed over 180,000 people in the United States, including more than 8,700 in Massachusetts.The virus is highly contagious, has a lengthy incubation period, and can be spread by symptomatic and asymptomatic individuals. There is no known cure, and no vaccine has been approved for public use.”

Healey also argues that Baker’s many orders stemming from his declaration of a state of emergency do not infringe on the powers of the Legislature, in part because Beacon Hill lawmakers have passed a number of laws on their own that incorporate and thus acknowledge as legitimate the state of emergency.

“Given the breadth and number of laws enacted since the governor’s emergency declaration, it blinks reality to suggest that the Legislature has been deprived of its authority to enact COVID-19-related laws. And should the Legislature disagree with any action taken by the Governor under the Civil Defense Act — including any COVID-19 order—it has multiple remedies, including one reserved within the Civil Defense Act itself: the power to make any part of the Civil Defense Act  “inoperative by the adoption of a joint resolution to that effect by the House and Senate acting concurrently.”