  Lumped-element kinetic inductance detectors (LEKIDs) are an attractive
technology for millimeter-wave observations that require large arrays of
extremely low-noise detectors. We designed, fabricated and characterized
64-element (128 LEKID) arrays of horn-coupled, dual-polarization LEKIDs
optimized for ground-based CMB polarimetry. Our devices are sensitive to two
orthogonal polarizations in a single spectral band centered on 150 GHz with
$\Delta\nu/\nu=0.2$. The $65\times 65$ mm square arrays are designed to be
tiled into the focal plane of an optical system. We demonstrate the viability
of these dual-polarization LEKIDs with laboratory measurements. The LEKID
modules are tested with an FPGA-based readout system in a sub-kelvin cryostat
that uses a two-stage adiabatic demagnetization refrigerator. The devices are
characterized using a blackbody and a millimeter-wave source. The polarization
properties are measured with a cryogenic stepped half-wave plate. We measure
the resonator parameters and the detector sensitivity, noise spectrum, dynamic
range, and polarization response. The resonators have internal quality factors
approaching $1\times 10^{6}$. The detectors have uniform response between
orthogonal polarizations and a large dynamic range. The detectors are
photon-noise limited above 1 pW of absorbed power. The noise-equivalent
temperatures under a 3.4 K blackbody load are $<100~\mu\mathrm{K\sqrt{s}}$. The
polarization fractions of detectors sensitive to orthogonal polarizations are
>80%. The entire array is multiplexed on a single readout line, demonstrating a
multiplexing factor of 128. The array and readout meet the requirements for 4
arrays to be read out simultaneously for a multiplexing factor of 512. This
laboratory study demonstrates the first dual-polarization LEKID array optimized
for CMB polarimetry and shows the readiness of the detectors for on-sky
observations.
