In terms of erosion of democratic rights by the Supreme Court, the gutting of the Voting Rights Act is the most literal example, as is the decision to prevent Al Gore from taking the White House in 2000 despite being the legitimate electoral victor. A more peripheral example is Citizens' United.
Non-judicial examples include sabotage of the USPS to prevent mail-in voting, the last state election in Georgia (where the then-secretary of state supervised an election in which he was a candidate; needless to say, he won), removal of polling stations in Democratic areas, targeted disenfranchisement of demographic groups that traditionally vote Democratic, Mitch McConnell rejecting measures to increase voter turnout on the grounds that making it easier to vote would be a "Democratic power grab," Republican use of big data to jerrymander safe districts to maintain GOP control of states despite losing the popular vote, FBI interference in the 2016 election by releasing unsubstantiated accusations (the Comey Memo) against Clinton shortly before the election, and, finally, the Republican party soliciting foreign help to illegally acquire and spread disinformation against Democrats.
These are not signs of a healthy democracy.
I can provide sources if you would like but this stuff is everywhere and simple DDG/Google searches about these examples should lead to plenty of information.