The White House has labeled critics terrorists, attributed violence to a left-wing conspiracy and investigated unions, nonprofits and other groups in a battle over free speech. A memorial to Charlie Kirk outside the Turning Point USA headquarters last year in Phoenix. The very public murder of Charlie Kirk on a Utah campus last September sent convulsions of grief through the political right.

Within days, the Trump administration began transforming the slain prophet of young conservatives from a martyr to a cause. Stephen Miller, a White House deputy chief of staff, said three days after the killing that Charlie Kirk’s last message was to dismantle radical left organizations fomenting violence. The message quickly turned into policy.

Now, a year later, it is clear how bright a line of demarcation the period after Mr. Kirk’s death has become in the efforts by President Trump and his allies to target Americans they see as “the enemy within.” The administration’s nearly exclusive crackdown on left-leaning organizations and activities has ushered in a pitched battle over political dissent and free speech. Officials have taken a governmentwide approach to scrutinizing citizens and organizations opposed to Trump policies. Tax agents have examined nonprofit groups.

Investigators have looked into the books of labor unions. Prosecutors have brought charges against anti-ICE protesters, asserting that a leftist conspiracy is provoking political violence on the streets. These efforts came together this year in Minnesota, the scene of heated protests in response to the government’s aggressive immigration sweep.

Federal investigators infiltrated neighborhood watch committees, church meetings and an environmental group in an operation that surveilled hundreds of ordinary people. Ultimately, 15 protesters were charged with conspiring to impede immigration enforcement as members of the far-left movement known as antifa, but prosecutors did not accuse them of any violent acts beyond kicking an ICE vehicle and sideswiping another. From the start of his second term, Mr. Trump has wielded power to punish those he views as opponents, including law firms, universities, pro-Palestinian activists and news outlets, three of which he banned last week from the White House.

But after the Kirk killing and assassination attempts on the president, Mr. Trump and his allies have recast the effort in existential terms. Andrew Kolvet, a spokesman for Turning Point USA, Mr. Kirk’s conservative organization, said in an interview that opponents on the left want them dead and they must deal with this or be a sitting duck. Mr. Trump issued two directives soon after Mr. Kirk’s death: on Sept. 22 last year, he signed an executive order designating antifa a domestic terrorist organization. Days later, he signed a national security document ordering an attack on networks he said fomented political violence.