Well, according to Article I, Sec. 8, the Congress has the following powers: "To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;
To provide for organizing, arming, and disciplining the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;"
So, I guess it would depend on the nature of the emergency, if this was legal. It would have to involve executing Laws, suppressing insurrection, or repelling invasion.
The insurrection act was changed in the 2007 appropriations act. To wit: “Section 1076(a): The John Warner Defense Authorization Act amended the quoted section (10 USC § 333), which is renamed Major Public Emergencies; Interference with State and Federal Law, to allow the President to employ armed forces, including the National Guard in Federal Service, “to restore public order and enforce the laws when, as a result of a natural disaster, epidemic, or other serious public health emergency, terrorist attack or incident, the President determines that “domestic violence has occurred to such an extent that the constituted authorities of the State or possession are incapable of maintaining public order.”
This should have been named the “Blanco - Pandemic Influenza” act.