Lawmakers and congressional aides from both parties were glued to television sets in Washington yesterday waiting for updates on the condition of Senator Johnson of South Dakota, who underwent several hours of surgery Wednesday night for cerebral bleeding.

The unusual interest in the health of a two-term senator was driven by the precarious, 51-seat Senate majority that Democrats won at the polls last month. If Mr. Johnson resigned or died, the governor of South Dakota, a Republican, would appoint a replacement and the Senate would likely return to Republican control.

Yesterday, the prognosis for Mr. Johnson, 59, was upbeat but guarded. “He has been appropriately responsive to both word and touch. No further surgical intervention has been required,” the Capitol physician, Admiral John Eisold, said in a statement after visiting the stricken senator yesterday afternoon at George Washington University Hospital in downtown Washington.

Senator Reid of Nevada, who is expected to become majority leader, was nearly ebullient about Mr. Johnson’s condition after visiting him yesterday morning.”He really looks good,” Mr. Reid told reporters. “He looked great to me.”

Journalists, skeptical that a man who had just undergone emergency brain surgery was in “great” shape, pressed for more details, but Mr. Reid would not elaborate. “Whatever I say about his medical condition would not be enough for you,” the Democratic leader said.

Mr. Reid dismissed suggestions that Mr. Johnson’s illness could upset the planned Democratic takeover. “There isn’t a thing that’s changed,” Mr. Reid said.

Still, word of Mr. Johnson’s hospitalization came as a blow to Democratic Senate aides who were gearing up to take over leadership of committees and the deliberative body’s top offices.

“The Democratic staff are all talking about it,” one Senate aide, who asked not to be named, said. “They’re concerned about his health, but they can’t help but be concerned about the political dynamics.”

Asked about the impact if Republicans retained control of the Senate while Democrats took over the House, the Democratic aide said, “If Democrats control only one chamber, the dynamic changes big time.”

However, one congressional analyst said little action is likely in the Senate, regardless of which party is in charge. “I’m really not expecting a lot to happen, particularly when half of the United States senators want to be president and are off in Nevada and South Carolina,” Stephen Hess of the Brookings Institution said.

With Mr. Johnson reported to be on the road to recovery, the prospects for the Republicans retaining control of the Senate seem slim. The South Dakota secretary of state, Chris Nelson, said there is no legal procedure for removing a senator who is incapacitated, unless the individual resigns or dies. “If there is a vacancy in the office of United States senator, the governor appoints a replacement to serve until the next general election,” which would be in 2008, Mr. Nelson said. If a Republican were appointed, the vote on organizing the Senate would likely be tied, 50–50, and Vice President Cheney could cast a tie-breaking vote, putting the body under GOP leadership.

Governor Rounds of South Dakota issued two short statements expressing hope for Mr. Johnson’s speedy recovery. “Our prayers are with Tim, Barbara, and their family. We are hopeful of good news for our friend and colleague,” the governor said late Wednesday.

Mr. Rounds, who was first elected in 2002, is considered fairly conservative, especially on social issues. Earlier this year he signed a bill that would outlaw most abortions in South Dakota. Voters repealed the measure in a referendum in November, but re-elected the governor to a second four-year term.

Many senators have served for years while infirm or in declining health. One often-cited example involves another senator from South Dakota, Karl Mundt. The Republican suffered a serious stroke in November 1969 and remained in the Senate through 1972.

“He never recovered,” a former press secretary to Mundt, Walter Conahan, recalled in an interview yesterday with The New York Sun. “He lost the use of his right side and didn’t have his speech.”

Democrats were in firm control of the Senate at the time, so a slight dip in the Republican count was not critical, Mr. Conahan said.

It was widely speculated that Mundt’s wife, Mary, said he would resign if she was appointed to replace him. “As far as I know, that’s never been verified,” Mr. Conahan said. Karl Mundt died in 1974 and his wife passed away in 1985.

Mr. Conahan said the senator was so addled after the stroke that he could not have been counted on in a floor vote. “I’m not sure he would have said, ‘Yes,’ if you needed a ‘yes.’ He might have said, ‘No,'” the former aide said.

Mr. Conahan said the episode pointed up the lack of a mechanism for removing a senator no longer physically fit for office. “In my view, he was not capable of resigning,” the former aide said.