Opponents of Donald J. Trump are drafting potential lawsuits in case he is elected in November and carries out mass deportations, as he has vowed. One group has hired a new auditor to withstand any attempt by a second Trump administration to unleash the Internal Revenue Service against them. Democratic-run state governments are even stockpiling abortion medication.

A sprawling network of Democratic officials, progressive activists, watchdog groups and ex-Republicans has been taking extraordinary steps to prepare for a potential second Trump presidency, drawn together by the fear that Mr. Trump’s return to power would pose a grave threat not just to their agenda but to American democracy itself.

“Trump has made clear that he’ll disregard the law and test the limits of our system,” said Joanna Lydgate, the chief executive of States United Democracy Center, a nonpartisan democracy watchdog organization that works with state officials in both parties. “What we’re staring down is extremely dark.”

While the Supreme Court on Thursday rejected an attempt to nullify federal approval of the abortion pill mifepristone, liberals fear a new Trump administration could rescind the approval or use a 19th-century morality law to criminalize sending it across state lines.

The Democratic governor of Washington State, Jay Inslee, said he had secured a large enough supply of mifepristone pills to preserve access for women in his state through a second Trump administration. The supply is locked away at a state warehouse.

“We have it physically in the state of Washington, which could stop him and his anti-choice forces from prohibiting its distribution,” Mr. Inslee said in an interview. “It has a life span of five or six years. If there was another Trump administration, it’ll get us through.”