The 40 Republican senators currently in the U.S. senate represent 36% of the U.S population. See the graph below (click on the thumbnail for PDF).
This is something I’ve been meaning to compute for a while now, mapping the cumulative distribution of senators’ ideal points onto the cumulative distribution of state population (each state counts twice, once for each senator, and of course we exclude DC and PR from the calculation of the total population).
With the relatively high degree of partisan polarization in roll call voting we place all 40 Republicans to the right of the Democratic senators. And — as recent roll calls on the health care issue have made clear — in the current 111th Senate the Democrats plus Sanders and Lieberman constitute 60 votes, with the most conservative Democrat (Nelson, NE) the “veto pivot”. Between them, the 60 Dems represent 64% of the population.
My prior expectation was for more “small state” bias pushing that 64% higher, but (a) Dems represent some small states (e.g., VT, ND, DE, MT, RI, HI have 2 Dem senators); (b) Republicans have the 2 TX and GA senators, and the OH/FL/NC Senate delegations are split.
The “bias” is a little more pronounced at the chamber median threshold: i.e., the 50 “most liberal” (Democratic) senators represent 57% of the population.