WASHINGTON, Dec. 14 (Xinhua) -- The control of the U.S. Senate in the new Congress set to convene early next month has been put into question with Democratic Senator Tim Johnson in critical condition Thursday after he underwent emergency surgery overnight.

    Johnson, 59, fell ill at the Capitol Wednesday and was rushed to hospital. He was now in the critical care unit at George Washington University Hospital.

    In a statement released by Johnson's office, Adm. John Eisold, attending physician of the U.S. Capitol, said the senator had "an intracerebral bleed caused by a congenital arteriovenous malformation. He underwent successful surgery to evacuate the blood and stabilize the malformation."

    He said it was "premature to determine whether further surgery will be required or to assess any long term prognosis."

    Speaking to reporters Thursday morning, Democratic Senator Harry Reid, who is set to become Senate majority leader when the 110th Congress convenes, said Johnson "really looks good" and that nothing has changed as far as Democratic control of the new Senate is concerned.

    "We're all praying for a full recovery," he said.

    Reid said he was confident that Johnson, a two-term Senate member, would recover, but he declined to disclose whether Johnson was conscious during his visit overnight. "I saw him; he looked great," Reid said.

    Some three weeks before the new Congress convenes, Johnson's illness created uncertainty over control of the Senate.

    Johnson was first elected to the Senate in 1996 after serving 10 years in the House. He was up for re-election in 2008.

    Democrats took control of the Senate from Republicans in last month's elections, but they would have only a very thin margin of majority in the new Congress.

    With Johnson in office, Democrats would hold a 51-to-49 edge. Should the senator die in office or decide not to serve out his term, South Dakota Governor Michael Rounds, a Republican, would name a replacement, presumably a Republican, for the next two years, news reports said.

    If that happens, the Republican Party would take control of the chamber, as in a 50-50 Senate, Vice President Dick Cheney, as the de facto president of the Senate, could break tie votes in the party's favor.