Scalise's office said he was in stable condition and undergoing surgery. President Donald Trump said in a tweet that Scalise was "badly injured but will fully recover."
Scalise, a representative from Louisiana, is the House of Representatives Majority Whip, making him the third-highest ranked member of the Republican leadership in the House.
In a dramatic account of the shooting, Representative Mo Brooks of Alabama said the shooter, who appeared to be a white man, was armed with a rifle.
Brooks told CNN he saw the man only for a second, and that he was shooting from a chain link fence behind the third base position on the field.
"There must have been 50 to 100 shots fired ... I hear Steve Scalise over near second base scream. He was shot," said Brooks, adding he helped apply a tourniquet with his belt to a congressional staffer who was shot in the leg.
"One of our security detail was shooting back, but it was our pistol versus the shooter's rifle," Brooks said. "The only weapon I had was a baseball bat."
Republican Senator Jeff Flake told local ABC-TV Scalise was shot in the left hip. Flake said the gunman was shot.
The Republican lawmakers were at an early morning practice ahead of an annual baseball game against Democrats that was scheduled to be played on Thursday.
"It's pretty well known in the neighborhood who those folks are on the baseball field," Brooks said. "It's not a secret we are practicing ... He was going after elected officials."
Scalise's position as whip means he has the difficult job of trying to keep order in the fractious party ranks and rounding up votes for bills.
'NO SHELTER'
David Miller, who saw the shooting, described a scene in which some players were far enough away from the gunman to run for shelter, while others were pinned down in the middle of the ball field.
"There was absolutely no shelter. They were in the middle of the ball park," he said. "They can't run. If they get up they are a target."
Trump, a Republican, said in a statement that he and Vice President Mike Pence were monitoring developments closely.
Democrats were also practicing for this week's game at another field at a different location, CNN reported.
Reba Winstead lives across the street from the parking lot of the park where the shooting occurred.
"I was on my front porch and that is when I heard the first round of shots. There was about a dozen shots. There was a pause. Then there was more shooting. I called 911."
Gabby Giffords, the last member of the U.S. Congress to be shot, was seriously wounded in a January 2011 assassination attempt at a gathering of her constituents in Tucson, Arizona. She survived, but six people were killed. Giffords resigned from Congress and became an activist for gun restrictions.
"My heart is with my former colleagues, their families & staff, and the US Capitol Police- public servants and heroes today and every day," she said in a Twitter message.
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