Liberals have spent years decimating norms of discourse. Pushing through a generational reform bill without half the country participating degrades the norms of democracy. When they lost Congress over this abuse, not only did they accuse Republicans of standing against the American people (even though the GOP kept expanding its majority) but said their position comprised nothing more than racism. Conservatives were no longer political opposition, they’re people who hate decency, democracy, the poor, the black, the infirm, America, and the system. As this thinking coagulated on the mainstream Left, Democrats had the moral justification to do what they liked.
Nearly the entire Obama presidency has been an exercise in figuring out ways to work around checks and balances. Unilaterally changing the status of millions of illegal immigrants because you can’t achieve your political goals may strike you as morally sound, but it oversteps any conception of executive power found in the Constitution. These people aren’t nervous about authoritarianism, they’re worried about how Trump would use it.
If you support a candidate like Hillary, who pushed the administration to get involved in the Libyan war without congressional approval, you’ll have little moral standing to be upset when Trump bombs people to “take their oil.” If you believe Obama has the right to assassinate suspected terrorists abroad without a trial, you have less authority to be upset when Trump threatens those associated with terrorists.