What do you think the federal government do if California seceeded.

Most likely the modern government would want to avoid a full on war. The first thing that would probably happen would be the Federal government taking command of the CA guard. Then, federal agents and/or troops would secure Sacramento and regulate the offices, policies, and bills produced there to keep them in line with the US. Ideally this happens without bloodshed. If there is significant bloodshed, the volatile state the nation is in at this moment could have large consequences. So if it happens in the near future, we'll probably see a muted response, at least at first, but then likely proceeding as described.