Nobel Prize-winning economist Paul Krugman told ABC News that President Obama has made more lasting changes to the country while in office than the last Democrat in office, President Bill Clinton.

"Bill Clinton is an incredibly gifted politician," Krugman told ABC News' Jonathan Karl. "But, in fact, Bill Clinton was not a consequential president. And Obama, although clearly not the natural politician, is a consequential president."

Krugman's comments follow a new cover story he wrote for Rolling Stone, in which he calls Obama one of the most "successful" presidents in American history.

His defense of the president comes as a surprise because Krugman has previously been a harsh critic of Obama. In 2009, Krugman was featured in a Newsweek cover story titled "Obama is Wrong: The Loyal Opposition of Paul Krugman," which identified him as the leading liberal voice against the president.

"People who had this idea that Obama was going to bring a transformation of America, I thought were being naïve," Krugman told Karl this week. "But, by God, we got health reform, and we got a significant financial reform. We are getting the environmental action … it's not everything you would have wanted, but it's more than anyone else has done for decades."

President Barack Obama speaks at a Town Hall Meeting at Cross Campus, Oct. 9, 2014 in Santa Monica, Calif.

Krugman also said that Obama's achievements were more consequential than those of Republican President Ronald Reagan.

"In the end, Reagan did not leave the structure of America's society particularly different," Krugman said. "He did not in fact change the basic legacy of Lyndon Johnson and FDR."

Krugman's ranking of consequential presidents, he said, would be Franklin Roosevelt, Lyndon Johnson, followed by Obama and then Reagan.

Krugman did acknowledge that national security is a weak area for Obama, who recently faced criticism from Leon Panetta, the president's former secretary of defense, and former President Jimmy Carter for his handling of ISIS.

"I don't think there's any way you can call him a great national security president," Krugman said.

"I don't think you can call him a terrible one either. And what I say is he's basically fairly typical for a post-Vietnam president," he said.

"Not everything is wonderful but some big achievements and no really huge disasters," Krugman said. "Obama really has left the world, has left America a different place."

Watch here: Paul Krugman Says U.S. Economy is On the Mend

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