In 1992, in Texas, the congressional Democrats agreed among themselves to go down as far as a 55/45 expected Democratic vote. In 2002, the Republicans agreed among themselves to go to 60/40. It just depends on how safe the incumbents want to feel–that is, how little re-election work do they want to put in. Obviously, the lower the “normal expected” partisan vote the more likely that an upset will occur for some reason (the economy, war, great opponent, scandal, etc.). Incumbents want to be in the majority, but they don’t always want to risk themselves to get there.