  We investigate the impact of cosmic rays (CRs) on the circumgalactic medium
(CGM) in FIRE-2 simulations, for ultra-faint dwarf through Milky Way (MW)-mass
halos hosting star-forming (SF) galaxies. Our CR treatment includes injection
by supernovae, anisotropic streaming and diffusion along magnetic field lines,
collisional and streaming losses, with constant parallel diffusivity
$\kappa\sim3\times10^{29}\,\mathrm{cm^2\ s^{-1}}$ chosen to match $\gamma$-ray
observations. With this, CRs become more important at larger halo masses and
lower redshifts, and dominate the pressure in the CGM in MW-mass halos at
$z\lesssim 1-2$. The gas in these "CR-dominated" halos differs significantly
from runs without CRs: the gas is primarily cool (a few $\sim10^{4}\,$K), and
the cool phase is volume-filling and has a thermal pressure below that needed
for virial or local thermal pressure balance. Ionization of the "low" and "mid"
ions in this diffuse cool gas is dominated by photo-ionization, with O VI
columns $\gtrsim 10^{14.5}\,\mathrm{cm^{-2}}$ at distances $\gtrsim
150\,\mathrm{kpc}$. CR and thermal gas pressure are locally anti-correlated,
maintaining total pressure balance, and the CGM gas density profile is
determined by the balance of CR pressure gradients and gravity. Neglecting CRs,
the same halos are primarily warm/hot ($T\gtrsim 10^{5}\,$K) with thermal
pressure balancing gravity, collisional ionization dominates, O VI columns are
lower and Ne VIII higher, and the cool phase is confined to dense filaments in
local thermal pressure equilibrium with the hot phase.
