As of today, conditions in the battle for Senate control are just about back to where they were on the day after the shake-up in the Kansas Senate race. Using polls alone in a 2-3 week window (see right sidebar), current medians show the following key margins: Alaska D+5%, Colorado D+2%, Iowa D+0.5%, North Carolina D+4%, and Kansas I+5.5%.
In an election based on today’s polled sample, the most likely outcome is 51 votes for Democrats and Independents.
[Update: see comments. At the moment, significant drivers of the difference between PEC and other sites appear to be (1) we're using all polls, including partisan ones, which changes Alaska; and (2) we're using Kansas two-candidate matchups and don't have fundamentals to drag those polls in the GOP direction.]
The first four races are critical for either side to win control, and among the closest of races this year. Money and time would be very well spent there, for both Democrats and Republicans.
There seems to be a fuss over today’s snapshot probability. It’s only there for rookie readers. The far more interesting and informative quantity is the Senate Meta-Margin, shown above. The Meta-Margin is defined as the amount of swing needed to create a perfect tie for Senate control.
In this calculation, Democrats+Independents=50 is scored as Democratic control because Democratic Vice-President Joe Biden, who presides over the Senate, would be able to cast a tiebreaking vote. If you do not like that assumption, subtract 0.83% from the Meta-Margin to assign Orman to the Republican caucus, or subtract 0.41% to split the difference.
The core assumptions of the Princeton Election Consortium calculations are: (1) to take a snapshot, accept all polls; and (2) the future will resemble the range of snapshots taken since June 1 (after accounting for the Kansas shake-up). Today’s change is an example of a swing toward the upper end of the expected range, which is between R+0.2% and D+2.5%, peak-to-peak.