After having lost ground in the Senate forecast for three consecutive weeks, Democrats have demonstrated improved polling in several key matchups over the past week, dimming Republican hopes for taking over the chamber.
The FiveThirtyEight model now gives Republicans an 18 percent chance of claiming control of the Senate after the Nov. 2 elections — down from 24 percent last week. The projected composition of the Senate has also changed slightly: more than 100,000 simulation runs of the forecast model show the Democrats finishing with an average of 52.0 senators, up from 51.5 last week, and Republicans with 47.9 senators, down from 48.4 last week.
...snip...
Control of the Senate, however, will boil down to a relatively small number of races — possibly not more than a half-dozen. And in several of those races, Democrats have made small, but important, gains:
...snip...
Instead, it is the 51st seat that poses the problem for Republicans. If Republicans were to win each of the races where they are currently favored, as well as the essentially tied races in Illinois, Nevada and West Virginia, they would emerge with control of exactly 50 Senate seats. But there is something of a gap between these states, and their next-best opportunities, which are in Washington and California — states where the consensus of polling suggests that Democrats would probably win if the election were held today.
http://fivethirtyeight.blogs.nytimes.com/2010/10/13/democratic-polling-improves-in-key-senate-races-lengthening-g-o-p-takeover-odds/