Authoritarianism experts worry that Trump could abuse his powers to try to disrupt the November election or delegitimize the result if he loses.
House Democrats called the incident in front of the White House this week a "direct threat to our democracy" in letter to Attorney General Bill Barr.
The helicopter, normally designated for use in medical evacuations, hovered low enough to create a deafening noise outside the White House.
"I went down, I looked at it," the president said of his time in the underground Presidential Emergency Operations Center.
“These attacks on peaceful American protesters steps from the White House were sickening and appalling," the Mass. senator said.
“I thought I was going to do two things: to see some damage and to talk to the troops,” Defense Secretary Mark Esper told NBC News.
"That was a provocation that was created deliberately for national television,” Sen. Marco Rubio (R-Fla.) said of Monday's White House protest.
Protests continued overnight across the U.S., hours after police fired tear gas at peaceful demonstrators to clear the way for President Trump.
“I reject the notion that the federal government can send troops into the state of Illinois,” Gov. J.B. Pritzker said.
“The president seems to think dominating Black people, dominating peaceful protesters, is law and order. It’s not."