Converting factfinder2.census.gov map data to KML?

$ ogr2ogr -f KML -s_srs EPSG:3857 -t_srs EPSG:3857 pop_by_census_tract.kml 140_00.shp

creates a KML file with the boundaries in the right places. It needs both of those options. Just -s_srs EPSG:3857 produces the same error as in the initial question, while -t_srs produces one like

ERROR 6: No translation for Mercator_Auxiliary_Sphere to PROJ.4 format is known.
Failed to create coordinate transformation between the
following coordinate systems.  This may be because they
are not transformable, or because projection services
(PROJ.4 DLL/.so) could not be loaded.
Source:
PROJCS["WGS_1984_Web_Mercator_Auxiliary_Sphere",
    ... # Same as 140_00.prj
Target:
PROJCS["WGS 84 / Pseudo-Mercator",
    ... # Same as in the gdal ticket