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 profits tax on oil companies to finance rebates for Americans.

At the heart of Mr. Obama’s proposals is a focus on fostering alternative energy development by investing $150 billion in emerging technologies and renewable fuels. Seeking to put a million fuel-efficient hybrid plug-in automobiles on the road, he said that he would offer a $7,000 tax credit to buyers, the overall cost of which he did not specify. In addition, Mr. Obama said his goal was to have 10 percent of the country’s energy needs met by renewable resources by the end of his first term, more than double the current figure.

While focusing on alternative energy production, Mr. Obama has veered in recent days toward increasing access to fossil fuels, both in seeking to tap the strategic oil reserve and in softening his opposition to offshore oil drilling. He said he might be willing to accept some exploration of limited offshore drilling as part of a more comprehensive energy bill that would include things he favors, like renewable fuels and batteries for electric-powered cars.