LANSING, Mich. — Senator Barack Obama altered his position on Monday to call for tapping the nation’s Strategic Petroleum Reserve to lower gasoline prices as he outlined an energy plan that contrasts with Senator John McCain’s greater emphasis on expanded offshore drilling and coal and nuclear technology. In a speech here and in a new advertisement, Mr. Obama, the presumptive Democratic nominee, also sought to portray his Republican rival, Mr. McCain, as “in the pocket” of oil giants that are profiting from gasoline priced at more than $4 a gallon. And in his speech, Mr. Obama called for a windfall...