There's a question on the SF-86

https://news.clearancejobs.com/2020/01/12/extremist-groups-and-your-clearance/

The vetting can be done as was done with the National Guard deployed to DC:

https://www.rand.org/blog/2021/02/what-the-capitol-siege-means-for-the-future-of-security.html

Even National Guard members deployed to serve as an extra layer of security at the inauguration were vetted by the FBI to ensure no infiltration by potential insider threats; 12 National Guard members were removed due to security concerns. One Army reservist with a current security clearance and access to a naval munitions depot in New Jersey was also arrested in connection with the assault on the Capitol.