Democracy is overrated when it devolves into mob rule. The Founding Fathers deliberately avoided making America a democracy, but rather established a careful balance of check and balances to avoid the inevitable abuse that comes from "majority rule".
Most important, in that design was the separation of powers between the branches of government, the difference with how the House and Senate are elected, the limitations of Federal powers, and several other clever mechanisms in our Constitution and Congressional rules (like the filibuster).
Nothing undermines our form of government more than when these checks and balances are undermined by a single party controlling to many branches of government. Ideally, the best way to find a balance between majority rules and individual rights is by having the two houses of Congress in different party hands.
This ensures that Presidents will be subject to Congressional investigations and legislation will be bipartisan. What we have now is a huge threat to our way of governance, the mere fact that Democrats openly considered eliminating the filibuster and packing the Supreme Court shows you how close we came to "one party rule".
Every voter should understand the best way to protect our form of government is to force bipartisanship. That can only happen if government is divided. If Democrats retain control of Congress and the Presidency who knows what machinations, they might put in place to preserve power indefinitely. To be clear, I am also not a fan when Republicans are in the same position.
Our only chance for a government to be responsive to all the people is to prevent a government that a simple majority controls.