NIDAL al-MUGHRABI and RAMI AYYUB PALESTINIAN militants fired more rockets into Israel's commercial heartland yesterday as Israel kept up a punishing bombing campaign in Gaza and massed tanks and troops on the enclave's border. The four days of cross-border violence showed no sign of abating and Israeli Prime Minister Benjamin Netanyahu said the campaign "will take more time". The violence has also spread to mixed communities of Jews and Arabs in Israel, a new front in the long conflict. Synagogues were attacked and fighting broke out on the streets of some communities, prompting Israel's president to warn of the danger…