It shouldn't surprise anyone that political maneuvering may get negative, this year it's assured.  From the Washington Post via MSNBC:

 Republicans are planning to spend the vast majority of their sizable financial war chest over the final 60 days of the campaign attacking Democratic House and Senate candidates over personal issues and local controversies, GOP officials said.

The National Republican Congressional Committee, which this year dispatched a half-dozen operatives to comb through tax, court and other records looking for damaging information on Democratic candidates, plans to spend more than 90 percent of its $50 million-plus advertising budget on what officials described as negative ads.

While policy & issue negative ads can be effective in Montana, personal attacks may be seen differently.  While its doubtful the GOP can dig up dirt on Tester, the veracity of any "controversy" will be highly scrutinized by voters.  While "personalizing" candidates negatively in other states may work, it will fail against Tester.  Tester has effectively taken that card away by his campaign's positive personal messages that are found within his campaign advertising.

Burns needs to focus on his base and moderate republicans.  Personal attacks will not bring those moderate R's who voted for Schweitzer back to his campaign.  They are exactly the group who will may more attention to the issues than any other.  A negative personal campaign attack would most likely solidify those cross-over voters squarely in Tester's camp.