Some appear to be interested in that question. One rigorous method - much more rigorous than National Journal's annual grab-bag - proposes the following:

By this method, Obama is liberal, but not that liberal. He was the 21st most liberal senator in the 109th Congress and has been the 10th or 11th most liberal thus far in the 110th. The surprising result is John McCain, who rates as the 8th most conservative senator in the 110th Congress, the 2nd most conservative in the 109th, and the 5th or 6th most conservative in the 108th.

But much more moderate in the 107th.

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