Washington (CNN) -- President Barack Obama outlined a plan Wednesday to cut America's imports of foreign oil by a third by 2025 -- a response to growing global energy demands and instability overseas.

The president's proposal relies primarily on increased domestic production, conservation, and a shift to biofuels and natural gas.

Among other things, Obama said he will push for an increased use of natural gas in trucks and buses, as well as the construction of commercial-scale biofuel refineries over the next two years.

The president also announced that he is directing the federal government to ensure that all of its vehicle purchases are alternative fuel, hybrid, or electric cars by 2015.

Higher fuel efficiency standards for cars will be announced this fall, he said. This summer, a new fuel efficiency standard will be proposed for heavy-duty trucks, he added.

The White House previously announced a decision to raise fuel efficiency standards for automobiles at a faster pace than required under the 2007 energy act -- boosting them to 36 miles per gallon by 2016.

"The United States of America cannot afford to bet our long-term prosperity and security on a resource that will eventually run out," Obama told an audience at Georgetown University. "We can't afford it when the cost to our economy, our country, and our planet is so high."

"There are no quick fixes," he warned. America "will keep on being a victim to shifts in the oil market until we get serious about a long-term policy for secure, affordable energy."

Obama said the country has to break the "political gridlock and inertia that's held us back for decades."

Despite the ongoing crisis at Japan's Fukushima Daiichi nuclear facility, Obama reaffirmed his support for a clean energy standard requiring utilities to buy 80% of their power from "cleaner" sources like wind and solar, as well as nuclear, natural gas and clean coal, by 2035.

"We can't simply take (nuclear power) off the table," he insisted.

In a swipe at Republican leaders, Obama said the 2008 GOP rallying cry of "drill baby drill" -- a reference to the push for more domestic oil drilling -- would do little to provide short-term price relief.

Nevertheless, the president stressed, America will have to increase its domestic oil production. Thirty-nine new shallow-water drilling permits and seven deep-water permits have been approved since tighter regulatory standards were imposed in the wake of last year's Gulf oil disaster, he noted.

Claims that the administration is responsible for rising oil prices might "make for a useful political sound bite," but they don't "track with reality," Obama insisted.

America only possesses 2% of the world's known oil reserves -- a reality that will require an eventual transition away from oil, he argued.

Obama ripped GOP budget proposals that would strip money away from his renewable energy agenda.

"Sacrificing these investments would weaken our energy security and make us more dependent on oil, not less," he said. "That's not a game plan to win the future. That's a vision to keep us mired in the past."

Senate Minority Leader Mitch McConnell, R-Kentucky, fired back, arguing that Obama "is telling people what he thinks they want to hear."

"Over the past two years the administration has taken what can only be described as a war on American energy," he said. "It's canceled dozens of drilling leases, it's declared a moratorium on drilling off the Gulf Coast (and) increased permit fees. ... In short, it's done just about everything it can to keep our energy sector from growing."

"Tell a Democrat in Washington that gas prices are too high and as if on cue ... they will take you on a tour of some alternative car plant that promises to have one of its $100,000 prototypes to market 25 years down the road," he said.

Sen. John Cornyn, R-Texas, released a statement arguing that "the American people need action from President Obama, not another lecture, especially as skyrocketing gas prices are crippling middle-class families and small businesses."

Obama's policies "have exacerbated the economic pain Americans are feeling at the pump," Cornyn argued. "It is time for the president to recognize the damage his policies are having on the economy, take his boot off the neck of domestic energy producers, and unlock our domestic energy potential."

Some conservative critics have accused the administration of doing little to stop rising gas prices as part of a strategy to speed a transition away from fossil fuels.

Energy Secretary Steven Chu dismissed those claims Wednesday, arguing that he he doesn't "think anyone can seriously consider that anyone in public office, especially the president, is cheering on" high gas prices.

CNN's Steve Hargreaves contributed to this report.