Much of the weakness in the Republican position comes from seven G.O.P. incumbents in mostly liberal-leaning states trying to win a second term after first being elected in the Republican wave year of 2010: Ron Johnson in Wisconsin, Mark Kirk in Illinois, Kelly Ayotte in New Hampshire, Pat Toomey in Pennsylvania, Roy Blunt in Missouri, Rob Portman in Ohio, and Marco Rubio in Florida (assuming he wins the Republican primary next week).

In addition, the retirement of the Republican Dan Coats along with the entry of the former Democratic senator Evan Bayh in the race in Indiana shifted that seat from a likely Republican hold to a possible Democratic pickup. Richard Burr’s seat in North Carolina also seems vulnerable.

Polling indicates that even Senator John McCain of Arizona could be swept out by the encroaching Democratic tide. If Mr. McCain beats back a primary challenge from the Tea Party candidate Kelli Ward, he will face a Democratic challenger, Ann Kirkpatrick, who was within two points in a June poll. Our model gives Ms. Kirkpatrick a roughly one-in-four chance of taking the seat.