NEW YORK, March 18 (Reuters) - Standard & Poor's said on Monday it revised its credit outlook on Venezuela to negative, indicating that a ratings downgrade may be on the way if critical economic reforms are held hostage to the political tension gripping the country. ``The current situation has led to political polarization and a sense of frustration among the population at large, including the business and labor sectors, the Catholic church, and the military,'' S&P said in a statement. ``This, in conjunction with presidential statements about the possibility of nationalizing banks ... and the danger of exchange controls or...