South Dakota Sen. Tim Johnson remained in critical condition today after undergoing emergency surgery for bleeding in his brain - an illness that could have immense political implications. They say that he has a congenital arteriovenous malformation - an abnormally tangled network of arteries and veins in his brain that ruptured, causing the bleeding. He is a modest, quiet man in an institution full of egos and sharp elbows, and has a mostly liberal voting record and a moderate demeanor. Beyond praying for a friend, Democrats may also be praying to keep their narrow 51-49 Senate majority. If Johnson's Senate seat were to become vacant, South Dakota law would empower Republican Gov. Mike Rounds to appoint a replacement. He would likely pick a fellow Republican, creating a 50-50 Senate with Vice President Cheney as the tie-breaker, leaving the GOP again in control of the Senate.