Currently you see many conservatives repeating the meme that losing the Senate means a Republican will never win the White House again. The belief being that Democrats will change voting laws, open borders and amnesty to Dem-leaning immigrants, and possibly even pack the Supreme Court and add DC and Puerto Rico as new states.
On the flip-side, similar sentiments were often expressed on the left of Trump heralding the end of American democracy. Republicans in power would permanently end fair elections through a combination of foreign election interference, gerry-pandering, dark corporate money, and voter suppression.
Disregarding whatever merits these views have, the fact that they're widely held by a significant chunk of the citizenry creates a dangerous equilibrium. Like you say, in a healthy democracy the losing side focuses its energy on winning the next election.
But if you think the other side's gonna use dirty tricks once in power, there is no next election to win. So you go all out it to stop the other side, and start looking for dirty tricks of your own. Now you've justified your opponents' bad opinion of you, and given them even more reason to win by any means unnecessary.