Ill Senator Is Called Responsive; Capital Is Riveted
Sen Tim Johnson is said to be in critical condition but 'responsive' after operation to stop bleeding in his brain; Democrats say they still expect to assume majority of Senate in January; Johnson's bleeding was caused by rare tangling of blood vessels in brain, and doctors say that in majority of cases bleeding does not cause long-term damage, though outcome in this case might not be known for weeks; Johnson would lose seat only if he died or resigned, upon which South Dakota Gov Mike Rounds would choose replacement until later special election; even if Johnson is unable to vote in January, Democrats would have 50-49 majority in Senate, which would allow them to take control of committees; photo (M)