Organizing • … there are groups that attempt to mobilize: Indivisible. MoveOn. People's Action. Center for Community Change. PICO. Center for Popular Democracy. Working Families Party.
Legal • Trump has already tried to stretch his powers beyond the usual limits. The Brennan Center, the ACLU and the Southern Poverty Law Center counter him.
Media • To combat Trump's alternative facts, subscribe to your local newspaper. Contribute to NPR. Even buy the "failing" New York Times if you must.
Party • The Democratic National Committee leadership is so weak that it may not be worth your time. But state and county party committees could use help. Skocpol recommends that if you live in a "blue" state, get your local committee to form a partnership with a party committee in a swing or red state.
Politics • If you'd like to run for office, consult your party, or Emily's List (if you're a woman), and train with Wellstone Action. For everybody else, find out where your labors are needed. There are gubernatorial races this year in New Jersey and Virginia. Flippable.org will lead you to important state races. Swingleft.org finds you the nearest congressional swing district in 2018.
Coordination • The left is desperately in need of people to align its identity-politics factions. America Votes and State Voices are attempting, against long odds, to do that.
Join • A church or synagogue, a union, your local Planned Parenthood chapter, the Chamber of Commerce, the Sierra Club, Elks Lodge, Veterans of Foreign Wars or American Legion post: The issue and the ideology don't much matter; what matters is connection.
And, of course, write letters to your local paper particularly if you have a Republican representative.