Electron confinement in the plug regions of thermal-barrier tandem mirrors is examined. Analytical and numerical (Fokker-Planck code) approaches are presented for small-to-intermediate mirror ratios including the effects of passing particles and electron heating. Steady-state trapped-electron densities and mean energies from the theory and code agree to within 20% for mirror ratios 0 ≤ R ≤ 2.2 with and without passing particles. Electron heating and detrapping by strong RF diffusion is demonstrated computationally for parameters expected in the TMX-U experiment.