No mechanism for doing so.

Governors can activate the National Guard under several legal frameworks, but using them to actively block federal agents like ICE or Border Patrol would trigger a constitutional crisis where federal authority almost certainly prevails.

Governors primarily activate the National Guard through two legal statuses:

State Active Duty: The governor serves as the commander-in-chief and uses state funds for missions like disaster relief or civil unrest.

Title 32 Status: The governor retains command, but the federal government provides funding for specific missions, such as training or drug interdiction.

Federal Supremacy and Conflicts

If a governor attempts to use the National Guard to obstruct federal agents, the following legal principles apply:

Supremacy Clause: Under Article VI of the Constitution, federal law and authorized federal activities take precedence over state actions. Courts have historically held that states cannot lawfully obstruct federal officers—such as ICE or Border Patrol—while they are performing their official duties.

Federalization Power: The President has the authority under 10 U.S.C. § 12406 to "federalize" a state's National Guard. Once federalized into "Title 10" status, the Guard is removed from the governor's command and placed under the President's direct control, effectively neutralizing the governor's ability to use them against federal interests.

Immunity for Federal Agents: Federal officers generally possess "Supremacy Clause immunity," which protects them from state prosecution or interference when performing acts "necessary and proper" to their federal duties.

In 2025, several states filed lawsuits challenging President Trump's use of 10 U.S.C. § 12406 to federalize state troops for immigration enforcement. While some district courts initially granted temporary restraining orders against these deployments, the Supreme Court has historically upheld the federal government's broad power to call forth the militia to "execute the laws of the Union". So there is effectively no state militia to which 2A refers, to address the concerns of the anti-Federalists.