Burr leads Elaine Marshall 43-33, Cal Cunningham 44-32, and Kenneth Lewis 44-31. Two common themes drive those leads for Burr- Republican voters are much more unified than Democrats and independents are leaning toward Burr as well.
The Democratic candidates are still suffering from low name recognition. 29% of voters have an opinion of Elaine Marshall while just 17% do of Kenneth Lewis and 14% do for Cal Cunningham.
Against a generic Democrat, Burr is currently leading 42% to 35%, with 23% not sure.
Burr's numbers are not great, but they are also not terrible. The fact that Burr is currently the most endangered incumbent Republican senator has much more to do with a string of Senate retirees and the relatively bad political climate for Democrats than his own potential weakness. Other Republican senators who might have faced tough re-election races in states like Missouri, Ohio, Florida, Kentucky, and New Hampshire have all retired. David Vitter (LA) is effectively the only other incumbent Republican senator who will possibly have a tough re-election, but, so far, he seems to be in fairly good shape.
The fact that Burr is possibly the only sitting Republican senator who might lose his seat, and is still polling ahead of his potential Democratic opponents, could partly explain why the Republican Senate leadership has been so successful in holding their caucus together for their strategy of complete obstructionism.