Clergy from Christ Church of Alexandria sing a hymn to open a prayer service in the aftermath of a gunman's shooting of Republican lawmakers at a baseball practice, in Alexandria, Virginia, US, June 14, 2017. [Photo/Agencies]

The assailant, who had nursed grievances against US President Donald Trump and the GOP, fought a running gun battle with police before he was shot and later died.

Colleagues said Scalise had been fielding balls at second base at a local park in Alexandria, just across the Potomac River from the nation's capital, as the Republicans practiced for their annual game with Democrats.

He dragged himself away from the infield, leaving a trail of blood before fellow lawmakers could rush to his assistance. He was listed in critical condition, but his office said that before he underwent surgery for a wound in his hip, he was in good spirits and spoke with his wife by telephone. Trump visited Scalise on Wednesday evening at Medstar Washington Medical Center.

The shooter was identified as James T. Hodgkinson, a 66-year-old home inspector from Illinois who had several minor run-ins with the law in recent years and belonged to a Facebook group called "Terminate the Republican Party". He had been living out of his van in the Alexandria area in recent months, the FBI said.

Capitol Police officers who were in Scalise's security detail wounded the shooter. He later died of his injuries, Trump told the nation from the White House.

"Everyone on that field is a public servant," Trump said. "Their sacrifice makes democracy possible."

Lawmakers noted their good fortune in having armed protectors on hand - "Thank God," they exclaimed over and over - and said otherwise the shooter would have been able to take a huge deadly toll.

The events left the capital horrified and stunned and prompted immediate reflection on the current hostility and vitriol in American politics. Lawmakers called for a new dialogue on lowering the partisan temperature,and Trump urged Americans to come together.

Proceedings were canceled for the day in the House, and instead, Speaker Paul Ryan of Wisconsin and Democratic leader Nancy Pelosi of California issued their own calls for unity.

"An attack on one of us is an attack on all of us," Ryan said to prolonged applause.

Shortly after the shooting, Bernie Sanders, a candidate for the 2016 Democratic presidential nomination, said on the Senate floor that the shooter apparently was a volunteer for his campaign last year. Sanders said he denounced the violence "in the strongest possible terms".

Scalise, 51, the No. 3 House Republican leader, was first elected in 2008. The popular and gregarious lawmaker is known for his love of baseball and handed out commemorative bats when he secured the job of House whip several years ago.

The Associated Press