Story Highlights• Too early to make prognosis on Sen. Tim Johnson's condition, doctor says
• Johnson had brain surgery Thursday morning
• Senator suffered brain hemorrhage on Wednesday
• Hemorrhage a result of congenital condition involving blood vessels
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WASHINGTON (CNN) -- Sen. Tim Johnson, who underwent brain surgery Thursday morning, is responding to word and touch, his doctor said.
The Democratic senator from South Dakota suffered a brain hemorrhage on Wednesday.
Johnson, 59, was in critical condition Thursday morning after surgery, said David Boyd, a spokesman in the nursing supervisor's office.
Adm. John Eisold, attending physician at the U.S. Capitol, said Johnson's post-operative recovery has been "uncomplicated."
According to the senator's spokeswoman, Julianne Fisher, Eisold said the bleeding in the senator's brain was the result of pressure from blood vessels that are too close together, a condition known as congenital arteriovenous malformation.
Johnson was born with the condition, he said. (Interactive: What is congenital arteriovenous malformation?)
"He underwent successful surgery to evacuate the blood and stabilize the malformation," Eisold said. "The senator is recovering without complication in the critical care unit.
"It is premature to determine whether further surgery will be required or to assess any long-term prognosis."
Replacing an incapacitated lawmaker?
Johnson's incapacitation raised questions about his seat in the Senate, where Democrats will hold a 51-49 edge in January.
The party wrested both houses of Congress away from the Republicans in the November midterm election.
Should Johnson not be able to complete his term, which ends in 2008, South Dakota Gov. Mike Rounds, a Republican, could appoint his replacement. (Time.com: As a senator is felled, a January vote looms big)
Such a move could shift the balance of power in the Senate.
The 17th Amendment to the U.S. Constitution allows states to choose how they replace a senator should he or she die or resign. In 43 states, including South Dakota, the governor can appoint a senator of his or her choosing. (Interactive:Can Johnson be replaced?)
South Dakota Secretary of State Chris Nelson said the appointment would fill the vacancy until a general election could be held in November 2008.
There are no restrictions on whom the governor could appoint beyond meeting the legal requirements for Senate membership, Nelson said.
Although the definition of incapacitation is not spelled out in state law, Nelson said there would be "precedent at the federal level."
One precedent, however, is the case of Sen. Karl Mundt, also from South Dakota, who suffered a debilitating stroke in 1969. (Long-term senate absences)
He remained in office until January 1973, when his term expired.
Mundt offered to resign but only on the condition that the governor appoint his wife to fill the vacancy. The governor refused, and Mundt retained the Senate seat.
Referring to the question of what Johnson's condition will mean to the Senate, a Democratic leadership aide told CNN on Thursday that the Senate is "not changing hands any time soon."
Former Senate Democratic leader Tom Daschle, also a South Dakotan, said as he was leaving the hospital that "it looks encouraging."
There's "no need" for Johnson to relinquish his seat, Daschle stressed.
Meanwhile, Johnson's wife, Barbara, issued a statement saying: "The Johnson family is encouraged and optimistic. They are grateful for the prayers and good wishes of friends, supporters and South Dakotans. They are especially grateful for the work of the doctors and all medical personnel and GWU hospital."
Senate Minority Leader Harry Reid, who visited Johnson on Thursday morning, told CNN that Johnson "looked very, very good."
Reid, of Nevada, declined to answer specific questions about Johnson's condition.
South Dakota's other senator, Republican John Thune, spoke from Baghdad, where he is part of a delegation.
"Right now all of us in South Dakota, across our state, are hoping and praying for Senator Johnson -- for the best possible outcome," he said.
He added, "We're praying and giving our best possible thoughts to him and his family."
Rep. Nancy Pelosi, who will become House speaker in January, extended "warm wishes and prayers" to the Johnsons.
Johnson served in the House 10 years before moving to the Senate.
A lengthy road to recovery
Dr. Sanjay Gupta, CNN's chief medical correspondent, said the bleeding apparently is on the left side of Johnson's brain, the part responsible for speech and strength on the right side of the body. (Watch CNN's Dr. Sanjay Gupta explain how speech difficulties indicated a problem )
The senator was in the Capitol conducting a conference call with South Dakota reporters on Wednesday when "his speech pattern slipped off," Fisher said. Johnson began slurring his speech and having difficulty finding words.
"This is frustrating," Johnson acknowledged toward the end of the call.
Several factors can cause the tangle of arteries and veins to place pressure on the brain, Gupta said, including hypertension.
Johnson's recovery would depend on the exact location where the bleeding occurred.
"We're talking about a long road here," to recovery, he said.
CNN's Ted Barrett and Dana Bash contributed to this report.