I don't if the term "resistance," with or without a capital R, will catch on for the anti-Trump opposition.
Peter Dreier likes the term, writing, "at the moment, our stance must be one of resistance and opposition." Trump Isn’t Hitler. But We Should Act Like He Is. Huffington Post 11/17/2016)
I'm very leery of Hitler analogies because they can lead to bad decisions based on inflating threats. And also because they can have a boy-who-cried-"Wolf!" effect.
Then again, we just elected Ross Barnett President of the United States.