With just a few days before Election Day, it has become increasingly clear that the Republicans are poised to take control of the House of Representatives. At the same time, however, the Republicans' chances of taking the Senate look to have faded somewhat as Democrats appear to be extending leads in races in California, Connecticut, and West Virginia. The momentum seems to be different in each chamber.
Part of this is the inherently different nature of House races and Senate races. Senate campaigns are typically trench warfare between well-funded, experienced politicians, who blast away at each other for the better part of a year in an attempt to move the polls. Once the primary concludes and the initial battle lines are drawn, there doesn't tend to be much movement until the end. House races, by contrast, involve lesser-known politicians with fewer funds, and tend not to engage until much later in the cycle.
Along the same lines, we have much better access to information about Senate races. People start polling them in the springtime, allowing us to gauge where they stand. But House races are notoriously underpolled. Everyone suspected that Baron Hill in IN-09 was in some degree of trouble this cycle, but we didn't get our first independent poll of the district confirming this until a week before Election Day.