There's no doubt that Democrats would have liked for Roy Cooper to be their candidate for the US Senate next year. But the news today that he's not running doesn't make Richard Burr that much less vulnerable than he was a week ago.
Like the Senate election in North Carolina last year, next year's contest is likely to be much more a referendum on Burr than his Democratic opponent. The fact that his approval rating is only 36% right now- and that 32% of likely voters in the state have no opinion about him- speaks to the fact that he is likely to have some of the same problems in standing for reelection that Elizabeth Dole did. His level of visibility has not been enough to make a positive impression with two thirds of the electorate, and he doesn't necessarily have a lot of accomplishments from his first term he'll be able to point to in making the case to the voters that he's been a strong advocate for them deserving of a second term.
The fact that Cooper's not running doesn't change the fact that only 29% of North Carolina's moderate swing voters approve of his job performance, with more disapproving. Or that even voters within his own party don't have that much enthusiasm for him- a 59% approval rating from Republicans.
Democrats showed last year that as long as they can recruit an attractive candidate with a story to tell that will resonate with the voters, initial name recognition isn't all that important. Kay Hagan started as an unknown statewide and trailed Elizabeth Dole 43-27 in the first poll where they were matched up.
Burr's vulnerability has a lot more to do with Richard Burr than it did with Roy Cooper, and as long as Democrats can get a reasonably compelling candidate today's news doesn't change the fact that Burr is one of the most endangered incumbents in the country running for reelection.