As the military presence ramped up in Los Angeles, communities are preparing for the largest protests against Trump since he took office.

For Vinnie Walsh, 82, of Auburn, Mass., it was “the whole nine yards — the loss of decency, courtesy, compassion, democracy.” Laurae Carpenetti, 54, a physician from the Atlanta suburbs, said that Mr. Trump’s appointment of Robert F. Kennedy Jr. as his top health official was her motivating factor.