A fight to streamline the Senates creaky rules may loom when the new Congress convenes next year. Majority Leader George J. Mitchell (D-Me.) is readying a reform package that would make it much easier to bring a bill to the floor. Under current procedures, a bill can be taken up only if no senator objects or if a majority approves a motion to proceed. So a single member can hold up legislation for weeks. And passing a controversial measure often requires breaking two filibustersone on the motion to proceed and another on the bill itself. Mitchell wants to move to a system like that used in the House, where legislation is taken up at the discretion of the majority leader. Trouble is, any change in Senate rules itself requires a two-thirds vote. And that will be difficult even if, as currently projected, the Democrats add a seat or two to their 57-43 margin in the fall elections.