GRAND ISLE, La.—BP PLC struggled for a second day Friday to stop the oil gushing from the area where the Deepwater Horizon blew up, as President Obama traveled here to try to assure residents the federal government is doing everything it can to protect them.

The president's visit to Grand Isle, a thin barrier island on the far southern tip of Louisiana about two and a half hours south of New Orleans, came as BP faced a day of stops and starts in its latest effort to stem the flow of oil. In the operation, known as a "top kill,"...