  The expanding ejecta from a classical nova remains hot enough ($\sim10^{4}\,
{\rm K}$) to be detected in thermal radio emission for up to years after the
cessation of mass loss triggered by a thermonuclear instability on the
underlying white dwarf (WD). Nebular spectroscopy of nova remnants confirms the
hot temperatures observed in radio observations. During this same period, the
unstable thermonuclear burning transitions to a prolonged period of stable
burning of the remnant hydrogen-rich envelope, causing the WD to become,
temporarily, a super-soft X-ray source. We show that photoionization heating of
the expanding ejecta by the hot WD maintains the observed nearly constant
temperature of $(1-4)\times10^4\mathrm{~K}$ for up to a year before an eventual
decline in temperature due to either the cessation of the supersoft phase or
the onset of a predominantly adiabatic expansion. We simulate the expanding
ejecta using a one-zone model as well as the Cloudy spectral synthesis code,
both incorporating the time-dependent WD effective temperatures for a range of
masses from $0.60\ M_{\odot}$ to $1.10\ M_{\odot}$. We show that the duration
of the nearly isothermal phase depends most strongly on the velocity and mass
of the ejecta and that the ejecta temperature depends on the WD's effective
temperature, and hence its mass.
