Johnson, 59, remained in critical condition. But Dr. John Eisold, the Capitol physician, said the South Dakotan was making an "uncomplicated" recovery and did not appear to require more surgery a day after being rushed to George Washington University Hospital from his office.
As Johnson lay in the critical-care unit less than three miles from Capitol Hill, senior lawmakers from both parties sought to downplay talk of a possible shift in power in the Senate. Still, Johnson's condition underscored the Democrats' tenuous hold on the chamber in the two-year congressional session that begins in early January.
If Johnson dies or resigns, South Dakota's Republican governor has the power to appoint a replacement that could wipe out the 51-49 majority the Democrats won in November's election.
Because a 50-50 tie would be broken by Vice President Dick Cheney, control of the Senate would revert to Republican hands.
Speculation about such a shift coursed through the Capitol after Johnson was taken to the hospital Wednesday afternoon when he became disoriented during a conference call with reporters.
Testing found internal bleeding in his head linked to a condition known as arteriovenous malformation, a congenital defect that produces a tangle of arteries and veins. In rare cases, the condition can prove fatal.
Emergency surgery Wednesday night relieved the bleeding, Eisold said.
In a statement released by Johnson's office, Eisold said that after surgery, the senator "has been appropriately responsive to both word and touch. No further surgical intervention has been required."
Johnson's wife, Barbara, said in a statement that the family was "encouraged and optimistic" about her husband's recovery prospects.
Because the senator made it through the operation without incident, his odds of survival are high, experts said. However, it is unclear whether or to what extent he may be impaired.
Those with arteriovenous malformation in the brain can suffer physical weakness, speech problems or paralysis, depending on where in the brain the malformation is located.
Even if Johnson is permanently incapacitated, history suggests there is little chance he would have to relinquish his seat. The Senate has never forced a member out of office for health reasons, even when senators spent years unable to make it to the floor to cast a vote.
The political consequences if Johnson's seat were to become vacant fueled a scramble for updates on his health and a rush to analyze any arcane Senate rule or South Dakota law that could come into play in replacing him.
If a Republican took over the seat, Sen. Mitch McConnell of Kentucky presumably would become majority leader, and the GOP would continue to determine the issues considered by the chamber. Democrats would still control the House, which the party won in November's vote, but Democratic hopes of a Congress united in challenging President Bush's policies -- especially concerning the war in Iraq -- would be dashed.
Sen. Harry Reid, the Nevada Democrat slated to become majority leader when the next Congress convenes, traveled to the hospital Wednesday night shortly after learning of Johnson's health problem. Thursday morning, Reid was mobbed outside the Senate chambers by reporters seeking word on his colleague's condition.
Standing in the glare of television lights, Reid declined to give a prognosis. "Whatever I say about his medical condition would not be enough for you," he said. He added, "He looked very good."
Elsewhere on Capitol Hill, Senate librarians were sent digging into the congressional record for information on how the chamber operated when it was split 50-50 after the 2000 elections.
At that time, Republicans assumed control of the chamber and its committees because Cheney was the incoming vice president. But as the Senate convened in January 2001, leaders from both parties struck a power-sharing agreement in which representation on committees and staffing for those panels were split evenly.
That accord ended five months later, when Republican Sen. James M. Jeffords of Vermont decided to become an independent who caucused with Democrats. That gave Democrats control of the chamber.
Republicans won back the majority in the 2002 election and held it in the 2004 vote.
Johnson's health problem also focused attention on officials in South Dakota. Secretary of State Chris Nelson patiently fielded calls from reporters seeking interpretations of its election code.
The law clearly gives the governor the power to fill a vacant Senate seat, Nelson said. An election for the seat then would occur in November 2008.
Johnson's term is due to expire then anyway, and GOP Gov. Michael Rounds has been mentioned as a strong potential candidate for the seat.
Rounds did not say what he would do if Johnson's seat became open. The governor, like politicians in Washington, confined himself to issuing a statement expressing concern for Johnson's condition and wishing him a speedy recovery.
Johnson held South Dakota's lone House seat before winning election to the Senate in 1996. He barely won reelection in 2002, defeating Republican John Thune by fewer than 550 votes. Thune then beat Democrat Tom Daschle, the Senate minority leader, in 2004.
Johnson's close call has made him a top target for GOP strategists eyeing the 2008 campaign.
A soft-spoken centrist who has largely avoided the spotlight, Johnson gained some attention in 2002 when he voted for the invasion of Iraq at the same time his son was in the Army. Johnson's son went on to serve in both Afghanistan and Iraq.
Times staff writer Thomas H. Maugh II contributed to this report.