The U.S. homeland security chief promised federal aid to people and businesses ravaged by Hurricane Isaac as President Barack Obama was to visit Louisiana. \"We are part of a team to make sure Hurricane Isaac is put to rest as soon as we can for all those affected,\" Secretary Janet Napolitano said a day before Obama was to meet with local officials, tour storm damage and view response and recovery efforts before talking with reporters in St. John the Baptist Parish, 35 miles west of New Orleans, the White House said. Republican challenger Mitt Romney toured hurricane-ravaged regions of the Louisiana bayou Friday, a day after accepting his party\'s nomination for president. \"We know this is a big, big tough storm, but we\'ll work through this together,\" Napolitano said on the steps of Slidell City Hall on the northeast shore of Lake Pontchartrain in St. Tammany Parish, about 30 miles northeast of New Orleans. Seven people were reported killed in the storm -- five in Louisiana and two in Mississippi. At least 4,000 evacuees remained in Louisiana shelters and tens of thousands of electrical customers remained without power, The Times-Picayune of New Orleans reported. Napolitano said people without power were victims of a nationwide weakness in electrical grids too fragile to deal with tough storms. Improving that infrastructure will be a priority as a lesson learned from Isaac, she promised. Obama widened the federal emergency declaration Friday to allow individuals in the most devastated parishes, including St. Tammany, to apply for federal assistance. Napolitano also visited Mississippi\'s coastal Bay St. Louis Sunday, 30 miles southwest of Biloxi, making similar promises of help to Mississippians. \"Whatever it was,\" Napolitano told a packed firehouse conference room, \"it was big, it was slow and it was wet.\" The Federal Emergency Management Agency, the homeland security division that coordinates the government\'s disaster response, conducted damage assessments with Louisiana and Mississippi agencies Sunday.