The Washington Post’s Chris Cillizza sees more U.S. Senate seats in play–with more seats showing a “probability” to switch party control, favoring Republicans:

The simple truth is that over the past six to nine months, the Senate playing field has expanded to the point where there are now (at least) 15 races where a party switch is a real possibility — if not a probability.[…]
11. Colorado (D): Appointed Sen. Michael Bennet (D) survived a surprisingly strong primary challenge in the former state House Speaker Andrew Romanoff earlier this month and watched as the GOP establishment pick — former Lt. Gov. Jane Norton — lost to Weld County prosecutor Ken Buck. Democrats could barely contain their glee at the general election matchup and quickly began the work of painting Buck as too conservative for the state. Smart state Democrats, however, know that Buck is an able candidate who might actually be more problematic for Bennet in an outsider-favored year like this one. (Previous ranking: N/A) [emphasis added]

The Cook Political Report lists Sen. Michael Bennet’s seat as a toss up, with the Rothenburg Political Report concurring, classifying the race as a “pure” toss up.