Here's a bit from a John Piper paper that helps to understand the NP position. I didn't copy the rest , because I don't want to critique the NP yet...just hear what they're proposing.
E. P. Sanders is the main spokesman for the way Pharisaism is reinterpreted by the New Perspective. Here is the way N. T. Wright summarizes it:
[Sanders'] major point, to which all else is subservient, can be quite simply stated. Judaism in Paul’s day was not, as has regularly been supposed, a religion of legalistic works-righteousness. If we imagine that it was, and that Paul was attacking it as if it was, we will do great violence to it and to him. . . . The Jew keeps the law out of gratitude, as the proper response to grace—not, in other words, in order to get into the covenant people, but to stay in. Being “in” in the first place was God’s gift. This scheme Sanders famously labeled as “covenantal nomism” (from the Greek nomos, law). (What Saint Paul Really Said, pp. 18-19)
Wright agrees with this main thesis of the New Perspective: “Sanders . . . dominates the landscape, and, until a major refutation of his central thesis is produced, honesty compels one to do business with him. I do not myself believe such a refutation can or will be offered; serious modifications are required, but I regard his basic point as established” (Ibid, p. 20).
For example, Wright says that the boasting which Paul aims to exclude by the doctrine of justification by faith (e.g., in Romans 3:27) is not what we usually think it is.
This ‘boasting’ which is excluded is not the boasting of the successful moralist; it is the racial boast of the Jew, as in [Romans] 2:17-24. If this is not so, [Romans] 3:29 (‘Or is God the God of the Jews only? Is he not of Gentiles also?’) is a non sequitur. Paul has no thought in this passage of warding off a proto-Pelagianism, of which in any case his contemporaries were not guilty. He is here, as in Galatians and Philippians, declaring that there is no road into covenant membership on the grounds of Jewish racial privilege. (Ibid, p. 129)