You should submit I-140 first. No question asked about "党员" on 140 stage. Do it as soon as you can.
"党员" question will be on I485, which you will have to wait for a while to submit from now on.
Please search old posts by "Xiao Bai Tu" about 党员 question. Her opinion is official. But I know a lot of people just fill in "No, I was not 党员" and got their green card without any problem.