The President has called upon his supporters to be poll watchers. It doesn’t work that way. You can’t just show up at the polls on Election Day and declare yourself a poll watcher.
It is against the law.
What’s worse is that the people he’s encouraging are armed militants; militants, not militia. The US does not have a militia and private “militias” are not legally allowed to act as law enforcement.
Not for the state governments, and not for the federal government.
Georgetown Law has put together this link highlighting the applicable law to turn away any of these right-wing, fascist members of the Trump fan club and keep them from intimidating voters trying to exercise their rights at the ballot box.
The link will take you to the Institute for Constitutional Advocacy and Protection fact sheets for all 50 states explaining the laws in each state barring unauthorized private militia groups and what to do if groups of armed individuals are near a polling place or voter registration drive.