The US leader said he was "monitoring the developments closely" in a statement released via the White House.
In a separate Twitter post, he also called Mr Scalise a "true friend and patriot". A morning speech from the US leader was cancelled.
Vice President Mike Pence also tweeted that he and his wife were "praying...for a speedy recovery" for all the injured.
Mr Scalise was reportedly in stable condition at the George Washington Hospital, in the US capital, Washington DC.
The House of Representatives Majority Whip was expected to survive after the early morning attack at a park in Alexandria, Virginia.
Congressman Mo Brooks, who was on the field at the time of the attack said said Mr Scalise was playing at second base when shots were fired from near the third base dugout.
Audibly shaken, he told CNN that he saw at least five people shot. He was able to administer first aid to one of the victims by tying a tourniquet around their leg.
Congressman Rand Paul, who was playing in right field, added that he heard at least 50 to 60 shots fired and Mr Scalise was moving after he was shot, dragging himself into the outfield.
His senior role as Majority Whip mean that Capitol Police were present as a limited security detail during the practice. Two officers from the detail were shot by the gunman.
Senator Jeff Flake said one of the officers brought down the gunman, after the other one was wounded. The shooter was also taken to a nearby hospital.
The Republican Party team has been using the field or several years.
Lafayette Square, the public area on the north side of the White House, was closed off by police as a precaution.Reuse content