Referring to "national emergency" as "two very big words," he said designating the coronavirus crisis that way would allow him to quickly get $50 billion to states, territories and localities "in our shared fight against this disease."
Trump declared the emergency under both the Stafford Act and the even bigger National Emergencies Act to unlock sweeping executive power in response to a national crisis.
Trump had indicated to reporters in the Oval Office on Thursday that he was considering enacting the Stafford Act, a law that allows a president to unlock the powers of the Federal Emergency Management Agency or FEMA to assist states and municipalities in a time of crisis.
"We have very strong emergency powers under the Stafford Act," Trump said. "I have it memorized, practically, as to the powers in that act. And if I need to do something, I’ll do it. I have the right to do a lot of things that people don’t even know about."
Former Acting Homeland Security Undersecretary John Cohen, now an ABC News contributor, says the declaration "signals the administration is finally recognizing the significance of these circumstances and bringing to bear all available resources of the federal government to address it."
"This is an important step that based on current conditions should surprise no one -- the only surprise is that it wasn't done sooner," Cohen said.
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Here's what you need to know about the powers that give presidents the authority to declare national emergencies:
What is the National Emergencies Act?
Enacted in 1976, the National Emergencies Act authorizes the president to declare a "national emergency" whenever he or she decides it’s appropriate. It offers no specific definition of "emergency," so it’s entirely at the president’s discretion.
Once declared, the president avails himself or herself of dozens of specialized laws -- some of which have access to funds the president otherwise could not use. Essentially, the president is allowed to bypass Congress and reprogram funding already allocated by lawmakers toward whatever areas the head of the executive branch deems fit.
Under current law, emergency powers lapse within a year unless the president renews them. A national emergency can be re-declared indefinitely and that has been done frequently in practice.
What is the Stafford Act?
Enacted in 1988, the Robert T. Stafford Disaster Relief and Emergency Assistance Act or Stafford Act empowers FEMA to directly assist state and local governments during "natural catastrophes" and coordinate the nation's unified response.
"It basically cuts the red tape of Washington and allows the federal government to rapidly direct more resources at local levels needed to respond to the disaster," Cohen said.
The Stafford Act essentially triggers FEMA's financial and physical assistance so that more than $50 billion in federal funding set aside by Congress can be utilized to respond to a crisis, such as a natural disaster or pandemic like COVID-19. The funding may be used on anything from paying first responders overtime to building new medical facilities, among other measures, so that states and municipalities can mobilize more quickly.
Why are the powers being invoked now?
Cohen, who has overseen crisis management in his various homeland security positions, says the move signals that the White House is "finally" acknowledging the importance of the federal government's response to the coronavirus.
"It's not like suddenly they discovered an element of this crisis that they didn't know about yesterday," Cohen said. "What it does mean is that the public health experts in the federal government have gotten through to the President."
"The declaration today, while an important step, simply means that the highest levels of administration have come to understand the significance of this crisis," he said, "and that they're taking prudent steps to make sure that federal resources are available."
Didn't the government already declare an emergency?
The Department of Health and Human Services declared the coronavirus a public health emergency in late January and has since been the lead agency addressing the pandemic -- but Friday’s declaration unlocks the ability of FEMA to use the reserve fund in the government's response.
"While this virus poses a serious public health threat, the risk to the American public remains low at this time, and we are working to keep this risk low," HHS Secretary Alex Azar said in a statement at the time. "We are committed to protecting the health and safety of all Americans, and this public health emergency declaration is the latest in the series of steps the Trump Administration has taken to protect our country."
In combination with the Public Health Emergency that's already been declared, Friday's declaration allows Health and Human Services to waive some of the regulatory requirements attached to federal health services programs like Medicare, Medicaid and CHIP, such as the requirement that a doctor has to be licensed in the state where he or she is operating. This can all help to facilitate delivery of health care services.
More than 30 state leaders, as well as officials from Washington, D.C., have also separately declared states of emergency -- but with the Stafford Act unlocked, FEMA has the power to lead a more unified response across the country.
How often does this happen?
The most recent national emergency was declared by President Trump last year when he used the order to reprogram funds to upgrade security at the U.S.-Mexico border.
Historically, there have been 58 emergencies pronounced under the National Emergencies Act since it was enacted in 1976 -- and at least 31 of them are still in effect.
The national emergency declared following the 9/11 terrorist attacks, for instance, has been renewed every year since 2001.
One more thing
The National Emergencies Act was also enacted by President Barack Obama during the H1N1 influenza pandemic.
The Stafford Act is named for the late Vermont Sen. Robert Stafford who served in Congress for nearly twenty years and helped pass the law.
ABC News' Katherine Faulders and Alex Mallin contributed to this report.