The 51-year-old congressman from Louisiana was in critical condition after surgery at a Washington hospital for a gunshot wound to the hip. Three other people were shot, and two more suffered minor injuries.
The attacker later died of injuries sustained in a shootout with police.
Scalise is considered a rising star in President Donald Trump's Republican Party, a gregarious lawmaker who headed a conservative grouping in Congress before being elected by his peers as the House majority whip.
The whip is the member of leadership who ensures discipline and makes sure the party has enough votes, particularly on key issues.
He has been seen as a bridge between the stark conservatives in the caucus, many of whom swept into office in the past seven years, and more traditional establishment Republicans.
Scalise had to face down demands for his resignation in 2014 after admitting to addressing a gathering with ties to Ku Klux Klan former leader David Duke, a scandal which complicated the party's effort to distance itself from the specter of white nationalism.
Scalise apologised, saying "it was a mistake I regret."
Majority Whip Rep. Steve Scalise, R-La., speaks with the media on Capitol Hill, Wednesday, May 17, 2017 in Washington. Source: AP
But the issue for which Scalise has made no apology is gun control. He is a fierce supporter of the National Rifle Association, which gives him an A+ rating, and has voted repeatedly to expand gun rights in the United States.
Scalise "will continue fighting to protect every citizen's Second Amendment right to keep and bear arms," his website says.
Last month, Scalise, who has described the constitutional right to bear arms as "incredibly sacred," introduced legislation that would relax restrictions on interstate firearm sales, making it easier for law-abiding citizens to buy guns.
He has routinely opposed gun control legislation, even taking to the House floor in a controversial speech criticizing activists in April 2013, just four months after 20 elementary school children and six adults were shot dead in Newtown, Connecticut.
"We were all shocked and saddened by the murders at Sandy Hook. But I think what's also disappointing, is when you have these tragedies, unfortunately there are people -- Washington politicians -- that try to take advantage of those tragedies to then come behind and try to impose their agenda," he said at the time.
Rapid political rise
Scalise was born in New Orleans and graduated from Louisiana State University. He worked as a software engineer and marketing executive before being elected to the Louisiana state House of Representatives in 1995.
After a brief stint as a state senator, he headed to Washington after winning election in 2008 for the congressional seat vacated by Bobby Jindal, who became the Louisiana governor.
Scalise rose rapidly through the ranks, and quickly made a name for himself on conservative issues.
In his third term, he was elected chairman of the conservative Republican Study Committee, a group of more than 160 House Republicans.
Trump on US Congressman's shooting
The group has played a vital role in debate over spending and deficit reduction efforts, but came in for criticism for proposing directions that were opposed by party leadership.
In November 2012, after winning the RSC chairmanship, he said he wanted to pull leadership "as far to the right" as possible in order to enact conservative legislation.
Two years later, he stepped down from that role when he was elected chief Republican whip, a coveted position traditionally seen as a stepping stone to greater power in Congress.
