23. ... c4
Sturm (1484) vs Blackwhiteboard (1588)

23. ... c4
If I did 23. ... c4, what would happen is this..
After the pawn fork, your defense is still okay. Bc8 is not a good move because it is passive and it breaks up your rooks. And, quite honestly, the A pawn just isn't worth that kind of trouble at this point in the game. Just play Bb5.
This is my game from Nimzo-Defense Tournament.. I played black in this game.. Enjoy!