Leaving aside the GOP catastrophe that is Donald Trump, the Republican Party’s impact on democracy has been distressingly corrosive.
Trumps ascension is really the result of twin Republican stratagems, and they have finally lost control of the monster they’ve created.
The first tool they employed was Gingrich’s notion that the GOP had to demonize its opponents. The mad rampage of Kenneth Starr is probably the worst example of this, but the email and Benghazi “investigations” are very much related.
The second was the establishment of the right-wing echo chamber that has served its audience a steady diet of fabrications and hyperbole for 20 years or more. Much of the distrust of Hillary Clinton is a product of the right-wing media smear machine.
Now we have a sizable portion of the electorate that doesn’t merely have different ideas of government’s role in society, but believes that a Clinton presidency would be a true catastrophe for the country. They are angry and heavily armed zealots for a weird Randian world view that probably regards them as worthy only to be serfs. They subscribe to the notion that the Obama presidency is the worst thing that has ever happened to the country, despite the (pre-2010) fiscal and economic policies that have put the country in vastly better shape that it was when Obama was elected. They blame Obama for the failures of an obstructionist Congress that has accomplished next to nothing since 2010.
Paul Krugman recently pointed out that Trump supporters don’t believe government data that the economy is better, yet report that they personally are doing better economically, which would seem to corroborate the government data, ain’a?
But, returning to the GOP’s assault on  democracy, we should not forget the egregious gerrymandering that has been conducted by GOP state legislatures. The GOP has lost 4 of the last six presidential elections, yet has won control of Congress, suggesting either that Americans are schizoid voters, or that Democratic votes count for less at the  congressional district level due to gerrymandering.
An important consequence of this gerrymandering is that Congressional incumbents are unassailable, and hence less accountable to their constituents. This is a large source of the contempt Americans have for Congress.
If the Democrats win the White House and the Senate, it will be a confirmation of the impact of gerrymandering, whose effect is primarily on the House of Representatives and state houses
It isn’t yet clear what impact the Trump candidacy will have on this state of affairs, but I’d guess that Paul Ryan’s career as Speaker of the House is doomed, and just maybe GOP control of state legislatures will be eroded in both the 2016 and 2018 elections. This could mean the reversal of gerrymandering and a return to a much more competitive political environment, which will force some honesty on the GOP.