Finish the fragment "build on the_____"?
For an example situation, imagine someone's trying to start a new club. This club has been gaining a greater number of new members everyday for 4 days. The club president knows she can't take a break from advertising for the club now, because she has to build on the _______, or else the opportunity will be missed. I know this word, but I'm sleep-deprived and I've drawn a blank that keeps getting longer as I try to force the word into my consciousness. It's driving me crazy. Thanks in advance :)