James Forsyth at The New Republic has an article online presenting the theory that a democratic victory in 2006 will aid republican chances is 2008 for the presidency. His argument essentially follows this path:
1. If the democrats claim a major victory in 2006, republicans will feel too insecure to nominate a more radical rightwing candidate (someone who is a clearcut social and religious conservative) and will be forced to nominate a more moderate candidate, or at least one with a more broadly based appeal. His examples for such a candidate are McCain, Rice, and Guliani.
2. Because studies showed that a "security gap" rather than a "values gap" caused Bush's reelection in 2004. Forsyth uses this logic to suggest that a moderate canidate can where while a far right conservative is less likely to acheive victory.
3. If the republcans hold congress, they will be emboldened into nominating a far right candidate unlikely to win in 08.
For discussion and reasons why this logic is flawed, see the flip.
There are several reasons why a democratic victory in november helps the democrats cause in 2008 as well, but the most glaring can be summarized in two words: Subpoena Power.
God only knows what the republican chaired committee's have refused to find in terms of congressional and presidential scandal. Until democrats can issue subpoena's to certain key players, we will never have a meaningful investigation of any number of important issues which help the democrats. These include, but are not limited to: Prewar intel, wiretapping, congressional ethic, and the unitary executive theory.
If these issues are not investigated, the furor over them will only last as long as the main stream media lets it, which means it will likely peter out when something more interesting comes along. Consider the Downing Street Memo. It was a major news story and is still relevant, but faded quickly out of the mainstream news. If democrats win the house, they can force issues embarassing to republicans to remain in the news. This will likely force the RNC to spend more money than it would like playing defense and less playing offense on the 2008 presidential and congressional elections. In addition to a financial gap, the nominee would be forced to shake off more "culture of corruption" baggage than he would if republicans control congress and thus can supress investigations of the above issues. Any gain democrats would have by the republicans winning in 2006 and nominating a far right politician for president would be offset by the fact that it would be more difficult for democrats to be on the offensive against the "culture of corruption" with subpoena's. A democratic victory in 2006 will help the democratic cause in 2008 by allowing a much more dramatic exposure of republican wrongdoing. If the republicans win, the news stories will generally focus on the fact that republicans were able to survive scandal and still be victorious. This will surely be the spin their victory would get, and this spin would be an excellent tool for Rovian's to dismiss further stories surrounding corrupt republicans as irrelevant. We cannot allow this to happen.