Dec. 14 (Bloomberg) -- Senator Tim Johnson underwent surgery to relieve bleeding in his brain and treat a congenital malformation of his arteries, his doctor said. Control of the new U.S. Senate may depend on Johnson's health.

Johnson, 59, is having an ``uncomplicated'' recovery, is responsive to word and touch and has not needed further surgery, Admiral John Eisold, the attending physician of the U.S. Capitol, said in a statement this afternoon.

Eisold said earlier today that Johnson ``was found to have had an intracerebral bleed caused by a congenital arteriovenous malformation.'' He underwent ``successful surgery to evacuate the blood and stabilize the malformation.''

Johnson's wife Barbara said in a statement that after the surgery, Johnson was responding to her voice, following instructions, and reaching for and holding her hand.

He is in critical condition, said an aide to Johnson who asked to remain anonymous.

Johnson, a South Dakota Democrat, was taken by ambulance to George Washington University Hospital in Washington yesterday afternoon after becoming disoriented during a conference call with reporters.

Democrats would have a 51-49 majority in the next session of Congress that begins Jan. 4 if Johnson retains his seat. If his seat were to become vacant, South Dakota Governor Mike Rounds, a Republican, may appoint a Republican replacement, shifting the partisan balance to 50-50 and giving Republicans control through the tie-breaking vote of Vice President Dick Cheney.

Split

The Senate had a 50-50 split between Democrats and Republicans following the 2000 election. The parties negotiated a power-sharing arrangement under which Republican senators chaired committees that had equal numbers of Democrats and Republicans. Leaders of both parties shared control of legislation reaching the Senate floor.

Democrats took the majority back five months later when Senator James Jeffords of Vermont quit the Republican Party and became an independent who voted with the Democrats.

Senate Democratic leader Harry Reid of Nevada, who recovered from a mild stroke last year, said Johnson looked good during a visit at the hospital and is confident he will have a full recovery.

``There isn't a thing that has changed,'' regarding Democratic control of the Senate in the next Congress, Reid said.

Confident

Johnson is one of 26 Democratic senators whose death or resignation would jeopardize Democrats' control of the chamber because their successor would be appointed by a Republican governor. That number will fall to 18 in January when the governorships of Arkansas, Colorado, Maryland, Massachusetts and New York switch to Democratic from Republican control.

House Speaker Nancy Pelosi said at a press conference today that Johnson ``is recovering and we are hopefully going to get some good news.'' She refused to address how his illness might determine control of the Senate.

First lady Laura Bush said she and President George W. Bush learned of Johnson's condition this morning.

``We're praying, like I know all the people of South Dakota are, for his very, very speedy recovery,'' she said in an interview on CBS's ``Early Show.''

Vacancies

While vacancies aren't uncommon in the Senate, they can only occur ``by death or resignation,'' said Richard Baker, the Senate historian.

``There either has to be a death certificate or there has to be a letter of resignation,'' he said. ``Nobody has the power to determine a vacancy for a person who is still living.''

Some lawmakers in the past have kept their seats in spite of long illnesses.

In 1969, South Dakota Senator Karl Mundt, a Republican, suffered a stroke that prevented him from voting, according to Slate, an Internet-based daily magazine. He kept his seat for three more years after South Dakota's governor refused his request to appoint Mundt's wife to replace him, Slate reported.

In the 1940s, Senator Carter Glass of Virginia, an 87-year- old Democrat, failed to vote for two years of votes due to illness while refusing calls to retire, Slate reported.

Johnson became disoriented while on a conference call with South Dakota and regional reporters at about 11:45 a.m. Washington time yesterday, according to Noah Pinegar, an aide to the senator.

Stutter

Johnson appeared to stutter in response to a question, then seemed to recover, asking if there were any additional questions before signing off, the Associated Press reported.

Johnson later walked with aides from the Capitol back to his office in the Hart Senate Office Building, a distance of about two city blocks, Pinegar said. Aides called the Capitol's attending physician, and Johnson was later taken by ambulance to the hospital. Johnson was conscious and talking throughout, Pinegar said.

Johnson went into brain surgery at approximately 7 p.m. Washington time last night and emerged at 12:30 a.m. today, according to Julianne Fisher, Johnson's spokeswoman.

``He came through the surgery and is doing well,'' she said.

The extent of impairment to Johnson's motor and cognitive functions is likely to become clear over the next two to three days, Fisher said.

``After having a surgery like this, the thing to do is wait,'' Fisher said.

Larry B. Goldstein, a Duke University neurologist, said it appears that Johnson was born with a malformation of brain blood vessels that burst, causing a type of stroke.

`No Symptoms'

``They can cause absolutely no symptoms for a lifetime or they can cause bleeding without warning,'' Goldstein, a spokesman for the American Stroke Association, said in a telephone interview today.

Johnson's surgeons would have performed an operation to remove the blood clot and seal off the weakened vessels using one of several approaches, Goldstein said. Strokes can alter speech, memory, personality, vision, gait, and the ability to move one's arms and legs.

``There could be no consequences or, if it's an important area of the brain and it's a large area, they could be severe,'' Goldstein said.

Johnson, an advocate for U.S. farmers and agricultural commodity producers, is in his second term in the Senate. He was re-elected in 2002 by just 528 votes out of more than 334,000 cast. An attorney, he had served five terms in the House of Representatives from 1986 to 1996 before winning his Senate seat. He is up for re-election in 2008.

To contact the reporter on this story: William Roberts in Washington at wroberts@bloomberg.net.

Last Updated: December 14, 2006 18:51 EST