President Trump on Wednesday said that the Justice Department would send federal law enforcement officials to Democratic-led cities experiencing an increase in violent crime, including Chicago, and again tried to draw a direct line between a rise in violence and Democratic policies.

“There is simply no substitute for a police department that has the strong backing of local leaders,” Mr. Trump said.

Though violent crime is down overall, major cities across America, including Atlanta, Chicago and New York, have struggled with an increase in murders and gun violence, including the shooting of 14 people near a Chicago funeral home on Tuesday.

While some local officials, including those in Kansas City, Mo., have requested help from the federal government, others have pleaded for those resources to be funneled toward other initiatives besides putting federal officials, or even troops, on the ground.

“I welcome legitimate resources from the federal government to reduce violence,” J.B. Pritzker, the Democratic governor of Illinois, told reporters earlier Wednesday. “But what I will not stand for are efforts that undermine civil rights and civil liberties like what’s happening in Portland. Any effort from the federal government to undermine the basic freedoms this nation has in its best moments aspired to protect will be met with resistance from this state government.”

Mr. Trump, whom polls show is trailing former Vice President Joseph R. Biden Jr., has sought to make law enforcement a central focus of his re-election effort by painting Democratic cities and policies as lawless.

By pushing forward with his federal response, named Operation Legend after LeGend Taliferro, a 4-year-old boy who was killed in June in Kansas City, Mr. Trump made what amounted to a direct move against several governors who have compared his decision to authoritarianism and threatened lawsuits, as well as Mayor Lori Lightfoot of Chicago, a Democrat who said Tuesday that “under no circumstances” would she allow federal troops to enter the city.

Operation Legend is separate from Mr. Trump’s recent decision to send federal agents to Portland, Ore., a move that has led to scenes of confrontations and chaos that, in turn, he and his White House aides have used to support a false narrative about Democratic elected officials allowing dangerous protesters to cause havoc.

Speaking after the president in the East Room, Mr. Trump’s attorney general, William P. Barr, tied the rise in violent crime in cities to protests that have demanded the defunding of police departments, and said that some 200 federal agents would be traveling to Chicago to work with local police.

“This rise,” Mr. Barr said, “is a direct result of the attack on the police forces and the weakening of police forces.”

Several cities that have attracted large-scale protests, including New York, Chicago and Minneapolis, have seen violent crime spike this summer, leading to a back-and-forth between police officers and activists about the root cause. Police officers in New York, for example, have said rerouting resources to protests caused a shortage in coverage in other areas, while critics have accused them of falling back amid increased scrutiny of inappropriate officer conduct.

But violent crime has also risen in Republican-led cities, a development covered less often by Mr. Trump and his advisers. Jacksonville, Fla., the site of the Republican National Convention next month, is experiencing one of its most lethal years in decades, with more than 100 homicides recorded.

In a coronavirus-focused briefing held later at the White House, Mr. Trump denied that his decision to send federal agents to Chicago was a political stunt. “The cities unfortunately that are in trouble are all run by Democrats,” he said. “You have radical left Democrats running cities like Chicago and so many others that we just had a news conference and unfortunately that’s the way it is.”