GovTrack's Political Spectrum & Legislative Leadership ranking
GovTrack has published ranking reports since 2004. The Political Spectrum & Legislative Leadership ranking is based on two factors - ideology and leadership. These are then plotted on a graph.
The political spectrum ideology score is based on of how Members of Congress cosponsor bills. Members who cosponsor in a similar manner are put next to each other on the left-right axis of the graph, while those that cosponsor different sets of bills are put at different ends.
Legislative leadership is also based on patterns of cosponsorship. As GovTrack explains, "Roughly speaking, each time a Member of Congress cosponsors another Member of Congress's bill,, the first Member of Congress gives the second Member of Congress some leadership points . . . in proportion to how many leadership points the first Member of Congress has."