WASHINGTON — A sizable portion of the American population has been convulsing with outrage at President Trump for more than a year. Millions of people who previously took only mild interest in politics have participated in protests, fumed as they stayed riveted to news out of Washington and filled social media accounts once devoted to family updates and funny videos with furious political commentary.

Yet public life on the whole has remained surprisingly calm. A significant factor in keeping the peace has surely been anticipatory catharsis: The widespread expectations of a big Democratic wave in the coming midterm elections are containing and channeling that indignation, helping to maintain order.

What will happen if no such wave materializes and that pressure-relief valve jams shut?

The country was already badly polarized before the plot twist of election night in 2016, of course, but since then liberals and much of what remains of America’s moderate center have been seething in a way that dwarfs the usual disgruntlement of whichever faction is out of power. While nobody can know for sure whether Mr. Trump would have lost but for Russia’s meddling, many of his critics clearly choose to believe he is in the White House because Vladimir Putin tricked the United States into making him its leader.

For Mr. Trump’s opposition, this premise — to say nothing of the question of whether his campaign conspired with Russia or merely benefited from its manipulations — has thickened the faint stink of illegitimacy that would hover over any president who lost the popular vote, supercharging policy disagreements into nearly existential threats to democracy. The regular cycles of consternation spun up by Mr. Trump’s unconventional approach to the job of being president help keep that wound raw.