Take note of a new phrase in the seemingly endless debate over whether the days of the United States as the world’s pre-eminent power are numbered: Those who doubt the country’s economic decline are holding an “intellectual ostrich position.”

The expression was coined by Edward Luce, author of a deeply-researched new book entitled Time to Start Thinking: America in the Age of Descent. It notes that the United States accounted for 31 percent of the global economy in 2000 and 23.5 percent in 2010. By 2020, he estimates that it will shrink to around 16 percent.

Luce’s diagnosis of descent, published in April, was the latest addition to a steadily growing library of books, academic papers and opinion pieces for or against the idea that the United States can maintain its status as the world’s only superpower. If we adopt Luce’s phrase, it’s a discussion between declinists and ostriches. The latter include President Barack Obama and his presumptive Republican rival in next November’s presidential elections.

“It means that we’re going to have a 2012 election where…both candidates will start on a false premise: that relative economic decline is simply to be ignored or dismissed,” Luce said in an interview with Foreign Policy magazine. “And I’d describe that as a kind of intellectual ostrich position.”

The false premise, in this view, was set out by Robert Kagan, a scholar at the Brookings Institution, a Washington-based think tank, in a lengthy analysis entitled Not Fade Away: Against the Myth of American Decline. One of the points Kagan made to support his argument: the U.S. share of world Gross Domestic Product (GDP) has held steady over the past four decades. Plain wrong, says Luce.