The assault on Speaker Nancy Pelosi’s husband, Paul, last week shocked even those who have become inured to rising violence in the United States. The erosion of norms restraining extreme behavior that began well before the election of Donald Trump in 2016 appears to have accelerated. Society looks as if it is coming apart at the seams.

The Reagan-era “government is the problem” language and ideology has been transformed into a philosophy that casts the government as not just a problem but as evil, a threat to the values MAGA supporters hold dear. Under Mr. Trump’s leadership, groups on the right have felt increasingly comfortable incubating, encouraging and carrying out violence.

The consistency of the rhetoric (“enemy of the people,” “Our house is on fire,” “You’re not going to have a country anymore,” “the greatest theft in the history of America,” “Where’s Nancy?”) has ingrained dehumanization of Republican opponents in parts of the political culture; conservatives have often painted their critics as enemies who must be annihilated before they destroy you. As the Department of Homeland Security has reported, domestic violent extremism — such as the white supremacist Charlottesville riots and the Jan. 6 insurrection — is one of the most pressing internal threats facing the United States.

Some on the left, too, have increasingly abandoned norms of civility and respect for rules and institutions. The gunman who in 2017 targeted Republican members of Congress and shot five people playing baseball — the Republican House whip, Steve Scalise, was seriously wounded — drew inspiration from his hatred of Republicans and Donald Trump. In June a California man was arrested outside Justice Brett Kavanaugh’s home and charged with attempted murder after the man posted on the social platform Discord that he was going to “stop Roe v. Wade from being overturned.”

What’s behind all this? While Democratic leaders for the most part are quick to condemn violence, Republican leaders increasingly minimize its severity or turn a blind eye. The tropes that Republican officials use demonstrate contempt for state authority, including law enforcement; a belief that un-American cadres have captured the government, cultural institutions and businesses; a pervasive distrust of the objective news media; an apocalyptic strain of thinking that America is in grave peril; and an animating acceptance of conspiracy theories and white supremacist sentiments. The peaceful transfer of power has conceded ground to a politics of the street.