We studied spin-polarized tunneling through a vacuum barrier using spin-polarized scanning tunneling microscopy on Co(0001). By varying the tip-to-sample distance in a controlled way, the tunneling magnetoresistance, i. e., the tunneling current asymmetry for parallel and antiparallel configuration of tip and sample magnetization, was measured as a function of the gap width. At large gap widths the asymmetry is constant. At gap widths below ≈ 4.5Å, a drop of the asymmetry was found in contrast to predictions of the Julliere model. The drop of the current asymmetry in spin-polarized tunneling is found to correlate with drop of the imaginary electron momentum inside the barrier and we compare our experimental results with the predictions of the Slonczewski model.