  By means of fully kinetic simulations, we investigate electron acceleration
during magnetic reconnection in a nonrelativistic proton--electron plasma with
conditions similar to solar corona and flares. We demonstrate that reconnection
leads to a nonthermally dominated electron acceleration with a power-law energy
distribution in the nonrelativistic low-$\beta$ regime but not in the
high-$\beta$ regime, where $\beta$ is the ratio of the plasma thermal pressure
and the magnetic pressure. The accelerated electrons contain most of the
dissipated magnetic energy in the low-$\beta$ regime. A guiding-center current
description is used to reveal the role of electron drift motions during the
bulk nonthermal energization. We find that the main acceleration mechanism is a
\textit{Fermi}-type acceleration accomplished by the particle curvature drift
motion along the electric field induced by the reconnection outflows. Although
the acceleration mechanism is similar for different plasma $\beta$, low-$\beta$
reconnection drives fast acceleration on Alfv\'enic timescales and develops
power laws out of thermal distribution. The nonthermally dominated acceleration
resulting from magnetic reconnection in low-$\beta$ plasma may have strong
implications for the highly efficient electron acceleration in solar flares and
other astrophysical systems.
