Democrats poised to lose control after Senator falls ill

Senior Democrats were in a state of high anxiety last night as a Senator remained in hospital after undergoing emergency brain surgery - his condition raising the prospect that the Republicans could regain control of the chamber just weeks after they lost it in the mid-term elections.

Senator Tim Johnson was said to be in a critical but stable condition after a late night operation to deal with bleeding on the brain. He had been taken to hospital on Wednesday after stuttering and becoming disoriented during a conference call and. Aides believed he may have suffered a stroke. Officials said yesterday that he has a congenital arteriovenous malformation - a condition where the sufferer is prone to internal bleeding.

Alongside the general concern for Mr Johnson, Democrats are worried about the implications of his condition and whether he will be forced to stand down. Democrats are preparing to return to Congress next month holding a 51-49 advantage in the Senate.

But if Mr Johnson resigns his position, his interim successor as Senator for South Dakota would be chosen by the State's Republican governor, Mike Rounds. If Mr Rounds were to chose a Republican candidate - as expected - the Senate would be split 50-50, but Republicans would hold the balance of power because, according to the Constitution, the Vice President, Dick Cheney, would hold the casting vote where issues are tied.

Officials said yesterday that Mr Johnson, 59, was stable after the surgery. The Capitol physician, Admiral John Eisold, said: "The senator is recovering without complication. It is premature to determine whether further surgery will be required or to assess any long-term prognosis."

The Senate Democratic leader, Harry Reid, said he had gone to the George Washington University Hospital to check on the condition of Mr Johnson, whom he described as a "dear friend to me and to all of us here in the Senate".

He added: "I was in the room with him. He really looked good. We are all praying for a full recovery."

South Dakota's Secretary of State said there were no special restrictions on Mr Rounds and that he could select a replacement from whatever party he chose to serve as an interim Senator for the last two years of Mr Johnson's term. An election for the seat is due to be held in 2008 and, ironically, Mr Rounds had been seen as the most likely Republican opponent to challenge Mr Johnson.

Mr Johnson's spokes-woman, Julianne Fisher, said he had become confused during a conference call with reporters. Before the call ended he appeared to have recovered and asked whether there were any additional questions. Ms Fisher said the Senator had then returned to his office in the Capitol, but appeared to be feeling unwell. The Capitol physician examined him in his office and recommended he go to hospital.

Mr Johnson was first elected to the Senate in 1996 after serving 10 years in the House. In 2002 he narrowly defeated the Republican John Thune. Two years later, Mr Thune beat Senator Tom Daschle, the former Senate Democratic leader.

A congenital arteriovenous malformation results in the creation of a mass of blood vessels - in Mr Johnson's case, in his brain. If they burst and bleed they can often kill before a person can get to hospital. Prompt surgery can tackle the problem. The condition is believed to affect about 300,000 Americans, according to the National Institute of Neurological Disorders and Stroke. It kills about 3,000 people a year.