America is “staring down the barrel of martial law” as it approaches the presidential election, a US senator from Oregon has warned as Donald Trump cracks down on protests in Portland, the state’s biggest city.

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In interviews with the Guardian, Democrat Ron Wyden said the federal government’s authoritarian tactics in Portland and other cities posed an “enormous” threat to democracy, while his fellow senator Jeff Merkley described it as “an all-out assault in military-style fashion”.

The independent watchdogs for the US justice and homeland security departments said on Thursday they were launching investigations into the use of force by federal agents in Portland, where unidentified officers in camouflage gear have snatched demonstrators off the streets and spirited them away in unmarked vehicles.

But Trump this week announced a “surge” of federal law enforcement to Chicago and Albuquerque, in addition to a contingent already in Kansas City. The move fuelled critics’ suspicions that the president was stressing a “law and order” campaign theme at the expense of civil liberties.

Wyden said in a written statement on Thursday: “The violent tactics deployed by Donald Trump and his paramilitary forces against peaceful protesters are those of a fascist regime, not a democratic nation.”

I wish the president would fight the coronavirus half as hard as he attacks my home town

Senator Ron Wyden

Speaking by phone, he said: “Unless America draws a line in the sand right now, I think we could be staring down the barrel of martial law in the middle of a presidential election.”

Military control of government was last imposed in the US in 1941, after the Japanese attack on Pearl Harbor that triggered entry into the second world war. In current circumstances it would entail “trashing the constitution and trashing people’s individual rights”, Wyden warned.

The Oregon senator recalled a recent conversation with a legal adviser for the head of national intelligence.

“I asked him again and again what was the constitutional justification for what the Trump administration is doing in my home town and he completely ducked the questions and several times said, ‘Well, I just want to extend my best wishes to your constituents.’

“After I heard him say it several times, I said my constituents don’t want your best wishes. They want to know when you’re going to stop trashing their constitutional rights.”

The White House press secretary, Kayleigh McEnany, began a briefing on Friday with a selectively edited video montage depicting protests, flames, graffiti and chaos in Portland.

“The Trump administration will not stand by and allow anarchy in our streets,” she said. “Law and order will prevail.”

Trump has falsely accused his election rival, Joe Biden, of pledging to “defund the police” so violent crime will flourish. Democrats condemn Trump for a made-for-TV attempt to distract both from Black Lives Matter protests and his mishandling of the coronavirus pandemic, now killing more than 1,000 Americans a day.

“I wish the president would fight the coronavirus half as hard as he attacks my home town,” Wyden said. “I think he’s setting up an us-against-them kind of strategy. He’s trying to create his narrative that my constituents, who are peaceful protesters, are basically anarchists, sympathisers of anarchists and, as he does so often, just fabricate it.

<span class="element-image__caption">Federal police under the orders of Donald Trump launch teargas after a demonstration in Portland, Oregon, on Thursday.</span> <span class="element-image__credit">Photograph: Anadolu Agency/Getty Images</span>
Federal police under the orders of Donald Trump launch teargas after a demonstration in Portland, Oregon, on Thursday. Photograph: Anadolu Agency/Getty Images

“Trump knows that his [coronavirus] strategy has been an unmitigated disaster. The coronavirus is spiking in various places and he’s trying to play to rightwing media and play to his base and see if he can kind of create a narrative that gives him some traction.”

The Portland deployment, known as Operation Diligent Valor, involves 114 officers from homeland security and the US Marshals Service, according to court documents. Local officials say their heavy-handed approach, including teargas and flash grenades, has merely enflamed demonstrations against police brutality and racial injustice. The justice department-led Operation Legend involves more than 200 agents each in Kansas City and Chicago as well as 35 in Albuquerque. It is targeted at violent crime.

Lori Lightfoot, the mayor of Chicago, has vowed to resist the federal intervention.

It’s very clear what the president is trying to do is incite violence and then display that violence in campaign ads

Senator Jeff Merkley

“We’re not going to allow the unconstitutional, state-sanctioned lawlessness we saw brought to Portland here in Chicago,” she said on Thursday.

Merkley offered warning words of advice based on Oregon’s current experience.

“I would say that you probably don’t believe that these federal forces will attack protesters if the protesters are peaceful and you will be wrong because that’s exactly what they’re doing in Portland,” he told the Guardian.

“This is an all-out assault in military-style fashion on a peaceful-style protest. The way to handle graffiti is put up a fence or come out and ask people to stop doing it, not to attack a peaceful protest but that’s exactly what happened. It’s very clear what the president is trying to do is incite violence and then display that violence in campaign ads. And I say this because that’s exactly what he’s doing right now. This is not some theory.”

The senator added: “This is just an absolute assault on people’s civil rights to speak and to assemble.”

Related: Chad Wolf: who is the Trump official leading the crackdown in Portland?

Merkley argued that with past targets such as Islamic State and undocumented migrants losing their potency, Trump has settled on African American communities in inner cities to be his latest scapegoats.

“I think it’s also important to note the president we’ve always known has this intense authoritarian streak,” he said. “He loved and had so much affection for the leader of North Korea, Putin in Russia. Just admiration for some of the tactics in the Philippines with Duterte and Erdoğan in Turkey, by the crown prince in Saudi Arabia.”

On Friday the United Nations warned against the use of excessive force against demonstrators and media in the US.

“Peaceful demonstrations that have been taking place in cities in the US, such as Portland, really must be able to continue,” the UN human rights office spokeswoman, Elizabeth Throssell, told reporters in Geneva.