Charlie Cook: a seven-seat gain would seem the most likely outcome for Senate Republicans.

The National Journal reports:

The odds of Republicans winning a Senate majority are obviously getting very high. Indeed, it would be a real shocker if Democrats held the GOP to a net gain of five seats or less, preventing a takeover of the majority. Unresolved, however, remains the question of just how big a night Republicans will have, and in turn, how to best interpret the results. At this point, at least in the Senate, this election seems to be a whole lot about the map, with more than a little political environment, including President Obama's current unpopularity, thrown in for good measure.

If Republicans take the Senate, but their gains are limited to the six states that Mitt Romney carried by at least 14 points—open seats in Montana, South Dakota, and West Virginia, along with the defeat of Mark Begich in Alaska, Mark Pryor in Arkansas, and Mary Landrieu in Louisiana (though this would not likely happen until a December 6 runoff)—that would be a good but hardly extraordinary night for the GOP. This would be a map election.

The establishment's Charlie Cook speaks.