The longitudinal form factors of the excited states J^π_f = 1/2^-, T_f = 1/2 and J^π_f = 7/2^-, T_f = 1/2 in ^7Li are calculated by using the 1p-shell model wave functions taking into account the core-polarization effects that connect the core state with higher energy configurations up to 32ħω through residual interactions. M3Y and MSDI are adopted as residual interactions. The best description of the data, including that beyond the minimum diffraction around q ≈ 3.3 fm^(-1), is obtained by using the MSDI residual interaction that excites intermediate 1p-1h states up to 18ħω.