Steve Scalise was one of four people wounded on 14 June by a gunman who targeted Republicans.
Doctors said they were concerned about infection, and he is now listed as being in a serious condition.
Mr Scalise, a 51-year-old who represents Louisiana, has been in hospital since the attack.
Prior to being shot he was the majority whip in the House of Representatives.
He arrived at hospital facing “an imminent risk of death”, doctors say, but was moved out of the intensive care unit in late June after repeated rounds of surgery.
The gunman, 66-year-old James T Hodgkinson, was shot by police at the scene and later died.