John McCain and Barack Obama have both called for changes to national energy policy in recent days that they said would eventually help lower oil and gasoline prices. On Sunday, Obama called for closer regulation of oil speculators that have been a major force contributing to the rise in oil prices. He said his plan would close the “Enron loophole” — a provision exempting trade in some energy commodities from government oversight, which allows speculators to run up costs. Obama also said he would work with other countries to increase regulation of their energy markets. On Monday, at a campaign stop in Fresno, Calif., McCain spoke about the need for “a swift conversion of American vehicles away from oil.” He called for a federal standard requiring more flex-fuel vehicles and said that the government should offer equal support to all forms of ethanol. McCain also said that better enforcement of existing fuel-efficiency standards is needed, and suggested that the federal government offer a $300 million prize to anyone who can develop a drastically improved car battery for use with hybrids or all-electric vehicles.