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...