Deuteron electrodisintegration cross sections near 180°have been measured near breakup threshold for the four-momentum transfer squared Q2 range 1.21-2.76 (GeV/c)2. Evidence for a change of slope in the cross section near Q2=1 (GeV/c)2 has been obtained. The data are compared to nonrelativistic calculations, which predict a strong influence of meson-exchange currents. The data are also compared to a hybrid quark-hadron model. None of these calculations agrees with the data over the entire measured range of Q2. The ratio of inelastic structure functions W1(Q2,Enp)/W2(Q2,Enp) is extracted from the present results and previous forward angle data. No prediction is in good agreement with the deduced ratios at small relative energy Enp.