Anti-Trump gunman attacks US lawmakers, dies in shootout
The 66-year-old gunman was identified as James T. Hodgkinson of Belleville, Illinois, an ardent supporter of liberal Senator Bernie Sanders - who said he was "sickened" by what he called a "despicable act."
In a national address, Trump appealed for unity following the morning assault in the Washington suburb of Alexandria, Virginia, which came amid high political tension in the US capital after a vitriol-filled 2016 election.
"We are strongest when we are unified and when we work together for the common good," said the president.
Congressman Steve Scalise underwent surgery after being shot in the hip and remained in critical condition, according to MedStar Washington Hospital Center.
Four others were wounded: a congressional aide, a lobbyist for Tyson Foods, and two US Capitol Police officers.
The FBI said it had recovered and was attempting to trace two weapons, a rifle and a handgun, and was actively investigating Hodgkinson's "associates, whereabouts, social media impressions, and potential motivations".
According to his Facebook page, Hodgkinson was a fervent supporter of Sanders, the feisty independent who battled Hillary Clinton for the Democratic presidential nomination last year.
Hodgkinson bashed Republicans for their health care and pro-rich policies, arguing for much higher taxes for the wealthiest Americans. He also branded Trump a "traitor".
"Trump has destroyed our Democracy. It's Time to destroy Trump & CO," he posted recently on Facebook with a Change.org petition against the president.
Sanders said in a statement that Hodgkinson "apparently" worked as a volunteer for his presidential campaign.
"I am sickened by this despicable act," Sanders said.
"Let me be as clear as I can be. Violence of any kind is unacceptable in our society and I condemn this action in the strongest possible terms."
Agencies contributed to this report.