The design parameters for high-performance, ultra-low-pressure water softening membranes based on a construct involving pore-filling microporous substrates with polyelectrolyte gels are presented. The design parameters were then used to make a series of gel-filled membranes by in situ crosslinking of poly(vinylbenzyl chloride) with diamines (piperazine and 1,4-diazabicyclo[2.2.2]octane) or poly(4-vinylpyridine) with dihalides such as alpha,alpha'-dichloro-p-xylene in DMF solutions in the pores of either a TIPS poly(propylene) or non-woven ultra-high molecular weight poly(ethylene) support. The separation properties of these membranes were tested under ultra-low pressure (100 kPa) using untreated municipal tap water. These membranes exhibit excellent separation properties, comparable to those of a commercial thin-film nanofiltration membrane (Desal-51, Osmonics), and productivity (flux) markedly higher than that of the thin-film membrane. The results of this study clearly show the importance of the mechanical strength (rigidity) of the microporous supports in designing high-performance pore-filled membranes.