House Majority Whip Steve Scalise, 51, was shot in the hip during the practice at a field in Alexandria, Virginia by the yet-to-be-identified rifle-wielding man, who was shot dead by the police.
Scalise's aides said the congressman was stable.
He was first elected to Congress in 2008, after serving as a member of the Louisiana state legislature for more than a decade. Scalise served as majority whip for roughly three years.
"Five people were transported medically from the scene," Alexandria police chief Michael Brown told reporters.
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The gunman was shot at by the police, apprehended and taken to a hospital. Later US President Donald Trump in a televised address from the White House announced the shooter has died.
He also praised the Capitol Police. "Many lives would have been lost, if not for the heroic actions of the two Capitol police officers who took down the gunman despite sustaining gunshot wounds during a very, very brutal assault."
About 25 senators and congressmen were practicing for more-than-a-century-old Congressional Baseball Game scheduled for June 15 at Nationals Park when the shooting took place.
The CNN quoting congressional and law enforcement sources said the shooting appears to be a "deliberate attack" and lasted for about 10 minutes.
Congressman Ron DeSantis, who left the scene just before the shooting began, said a man had asked him if it was Republicans or Democrats practicing as he walked to his car. "About three minutes later, at around 7:15 am the shooting began."
The shooting targeting Congressmen has shocked many in the US, where an intense debate is raging on gun control.
Earlier today, Trump in a statement said he was closely monitoring the developments in Virginia. "The vice president (Mike Pence) and I are aware of the shooting incident in Virginia and are monitoring developments closely," Trump said.
"We are deeply saddened by this tragedy. Our thoughts and prayers are with the members of Congress, their staffs, Capitol Police, first responders, and all others affected," the president said.
Senator Rand Paul described the scene as "sort of a killing field". "Without the Capitol Hill police it would have been a massacre," he told Fox News.
Congresswoman Nancy Pelosi called the shooting a "despicable and cowardly attack." "We are profoundly grateful for the heroism of the Capitol Police, whose bravery under fire undoubtedly saved countless lives. On days like today, there are no Democrats or Republicans, only Americans united in our hopes and prayers for the wounded," she said.
Indian-American Congressman Raja Krishnamoorthi called it a horrible, senseless violence.
"I was not at the scene today, but my thoughts and prayers are with Congressman Scalise, the Capitol Police officers and others who were shot, and their families," he said.
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