US Attorney General William Barr is to say attacks on federal buildings in Portland, Oregon, are “an assault on the government of the United States”.
In congressional testimony, America’s top law official will defend the Department of Justice’s decision to send security forces to the city.
Portland has seen 61 consecutive days of protest, which escalated after federal officers arrived this month.
Mr Barr has been accused by Democrats of politicising the justice department.
The Portland protests began as part of the nationwide racial justice rallies triggered by the death of George Floyd, an unarmed black man, in Minneapolis, Minnesota, in May.
According to his prepared remarks, Mr Barr will say in Tuesday’s testimony to the Democratic-controlled House of Representatives Judiciary Committee: “The most basic responsibility of government is to ensure the rule of law, so that people can live their lives safely and without fear.
“The Justice Department will continue working to meet that solemn responsibility.”
It will be his first appearance before the committee since becoming attorney general in February last year.
Mr Barr will deny he acted at the behest of US President Donald Trump, a Republican, when he interceded in criminal cases against his political allies – such as Roger Stone and Michael Flynn.
He will also accuse Democrats of “conjuring up a narrative that I am simply the president’s factotum who disposes of criminal cases according to his instructions”.
Democrats say Mr Barr has turned the Department of Justice into a political tool for the president, though he insists he maintains independence from the White House.
They have previously accused the attorney general of releasing a distorted summary of justice department special counsel Robert Mueller’s report on alleged Russian interference in the 2016 presidential election.
His department has also been criticised for sending federal officers to forcibly disperse protesters in Washington DC.
A National Guard major will dispute the White House’s account of that June demonstration in separate congressional testimony on Tuesday.
House Judiciary Committee Chairman Jerrold Nadler wrote this month: “Citizens are concerned that the Administration has deployed a secret police force, not to investigate crimes but to intimidate individuals it views as political adversaries, and that the use of these tactics will proliferate throughout the country.”
The protests had already been going on for weeks when federal agents were sent to Portland on 4 July to guard federal buildings.
Local officials say the demonstrations were peaceful until the federal agents showed up, but the justice department says that is not true.
The Mark O Hatfield Federal Courthouse in the city centre has become a nightly battleground.
Protesters have attempted to breach the fence that surrounds the building by climbing the structure or even using power tools, according to the Associated Press news agency.
A number of officers have been injured in the clashes as protesters have fired commercial grade fireworks over the barrier, pointed laser beams in the eyes of agents posted as lookouts, and hurled rocks and other projectiles over the fence.
Federal officers have responded with tear gas and less-lethal munitions that have injured several demonstrators.