You might want to remove the "just" from the topic title, since six is an improvement from the last Survey USA poll for McCaskill (there's one more recent than the weird +9 one).
SurveyUSA has always seen this race as more Democratic than anyone else, but Rasmussen really does appear all out on its lonesome.
Actually, earlier this afternoon SurveyUSA released a poll with McCaskill up nine.
The SUSA trendlines are now
10/23, McCaskill up nine.
10/30 McCaskill up three.
11/06 McCaskill up nine (not including Monday interviews).
11/06 (including Monday interviews) McCaskill up six.