(CNN) Rep. Steve Scalise has been readmitted to the intensive care unit "due to new concerns for infection," a statement from his office said.
His condition is listed as serious, according to the statement.
Scalise, the Republican House majority whip who represents Louisiana, was critically injured when a gunman, identified as James T. Hodgkinson, opened fire on the GOP baseball team as they were practicing for a charity game on June 14.
Hodgkinson, a small business owner in Illinois, had been vocal about his support for Bernie Sanders -- and his hatred for conservatives. He died following a shootout with authorities.
Scalise sustained a single gunshot wound to his left hip and suffered "significant damage" to his blood vessels, bones and some internal organs, his doctor said last month.
The congressman's condition subsequently improved, and he was released from the MedStar Washington Hospital Center's intensive care unit less than two weeks ago.
An aide to Scalise told CNN, "From the start, the whip's doctors have made it clear that this will be a process with ups and downs, and this is in keeping with those expectations."
Another update on Scalise is expected Thursday, according to his office.
Following the news of his return to the ICU, many tweeted well wishes using the hashtag #PrayForScalise.
Four people, including Scalise, were shot and two others were injured.
Matt Mika, a lobbyist for Tyson Foods and former congressional aide, was shot. His condition improved and he was discharged from the hospital on June 23.
Zach Barth, a congressional staffer, was shot in the leg and was released from the hospital shortly after.
Crystal Griner, a US Capitol Police officer, was shot in the ankle. She later tossed the first pitch at the Congressional Women's Softball Game.
David Bailey, a Capitol Police special agent, was treated for a minor injury after responding to the shooting, police said. The special agent threw the first pitch at the Congressional Baseball Game while walking on crutches.
The annual baseball game between Democrats and Republicans took place the day after, as both teams wore Louisiana State University hats in honor of Scalise for part of the game.
Texas Rep. Roger Williams sustained a leg injury and spoke to the media, tearfully thanking officers Griner and Bailey.