When Sen. Ben Nelson, D-Neb., announced his retirement, Democrats received a quick reminder of how bleak 2012 could prove for their Senate prospects. The retirement of Democratic Sen. Kent Conrad in North Dakota gives Republicans a near-gimmee. The open Nebraska seat, for which two strong Republicans are already in contention, could be a second. And that would leave Democrats no margin for error as they seek to defend against a GOP takeover of Congress’ upper chamber.