President Trump visited Scalise's bedside and spoke with his family in the hospital, later tweeting that Scalise was "a real fighter" and urging people to "pray for Steve". "Our thoughts and prayers are with the members of Congress, their staffs, Capitol Police, first responders, and all others affected", Trump said in a statement.
In an earlier announcement from the White House, Trump also praised the "heroic actions" of the Capitol Police officers who stopped the shooter in his tracks.
Two minutes later, the gunman, 66-year-old James Hodgkinson from the St Louis suburb of Belleville, Illinois, was down. He had been living out of his van in the Alexandria area in recent months, the Federal Bureau of Investigation said. The bullet caused massive bleeding, and damaged bones and internal organs.
The hospital said after the surgery that Scalise remained in critical condition.
Scalise, the House Majority Whip, is part of congressional leadership and among just a handful of members entitled to his own security detail. "A member of the U.S. Congress got shot because they didn't like [his] political views", Democrat Representative Tim Ryan said.
Lawmakers took cover in the dugout.
"If he had gotten on the field, we had nowhere to go", Williams said.
WARNING: Video below contains graphic language and imagery.
Lawmakers At Shooting Say Scalise's Security Prevented A 'Massacre'
A security detail accompanied Scalise to the field that morning - a factor many lawmakers at the scene insist saved their lives. NRA spokeswoman Dana Loesch praised the Capitol Hill police, saying that "good guys with guns kept this from getting worse".
Congressman Jeff Duncan of SC said he had just left the practice and encountered the apparent gunman in the parking lot before the shooting. The man calmly asked which party's lawmakers were practising and Duncan told him they were the Republicans. The man thanked him.
Trump and Melania also met Capitol Police Officer Crystal Griner and his wife.
The former minor-league baseball player has coached the GOP Congressional team since 2013, his office said. His condition is critical, and he will require additional operations.
According to reporting from CNN on Anderson Cooper 360, the Trumps were unable to actually see the wounded congressman.
"We may have our differences, but we do well in times like these to remember that everyone who serves in our nation's capital is here because, above all, they love our country", Trump said.