An emerging coalition that views Donald J. Trump’s agenda as a threat to democracy is laying the groundwork to push back if he wins in November, taking extraordinary pre-emptive actions.

If Mr. Trump returns to power, he is openly planning to impose radical changes — many with authoritarian overtones. Those plans include using the Justice Department to take revenge on his adversaries, sending federal troops into Democratic cities, carrying out mass deportations, building huge camps to hold immigrant detainees, making it easier to fire civil servants and replace them with loyalists and expanding and centralizing executive power.