National Guard in the COVID-19 Pandemic Response: Framework for Action [April 1, 2020] [open pdf - 433KB]
From the Document: "The Guard is an adaptable tool at the disposal of State Governors and, when 'federalized,' at the disposal of the President to respond to a range of threats to the security of the U.S. populace. [...] The Stafford Act allows the U.S. government to provide assistance to state and local governments after a disaster or during an emergency, to include funding National Guard state active duty for a state mission or FTNGD-OS [full-time National Guard duty for operational support] for a federal mission. DOD's DSCA [Defense Support of Civil Authorities] policy allows it to assist states if reimbursed by another federal agency, such as FEMA (DOD Directive (DODD) 3025.18)."
CRS In Focus, IF11483
Congressional Research Service: https://crsreports.congress.gov/