Charlie Cook says that Democrats might not want congressional majorities and thus modulate their efforts to try to get a gain but not enough to tip control of Congress. This may very well be right and Charlie Cook is famous as an electoral prognosticator for good reason.
The problem with the scenario is that electoral gains are not so easily calibrated. Both the Republican party and the people have a say as to how many seats the Democrats have come January. If the Democrats are so foolish as to not actually try their best, they may find that instead of a satisfactory gain that sets them up for 2008 majorities, President Bush's electoral reputation will get burnished again as he survives yet another mid-term without losing seats in Congress.