Is American power in decline, relative to the rest of the world? That question is at the center of a provocative study by the U.S. intelligence community exploring what the world might look like in 2030.

The answer, judging by comments from a panel convened to discuss the topic, is that America faces serious trouble: The U.S. economy is slowing, relative to its Asian competitors, which will make it harder for the country to assert its traditional leadership role in decades ahead. That, in turn, could make for a less stable world.

This pessimism among intelligence analysts contrasts sharply with the relentlessly upbeat prognostications made by politicians, especially the field of Republican presidential candidates, who describe an America of perpetual sunshine and unchallenged leadership. That’s certainly not the view of this nonpartisan group.

The unclassified study, titled “Global Trends 2030,” is being prepared by the National Intelligence Council, which is part of the Office of the Director of National Intelligence. This is the fifth such study (the first, published in 1996, looked toward 2010) and the only one to radically question U.S. staying power.

In preparing the document, the analysts decided to focus on America’s role in shaping the global future. “You have to be intellectually honest that there are changes in the U.S. role, and the role of rising powers,” that will affect events, explains Mathew Burrows, a counselor at the National Intelligence Council and the principal author of the report.

Burrows and other contributors met in Washington this month to hear outside comments — and it was an eye-opening discussion. A somewhat pessimistic paper on the U.S. economic outlook, prepared by Uri Dadush of the Carnegie Endowment for International Peace, was criticized at this meeting for not being pessimistic enough.

The base-line scenario offered by Dadush was that America would avoid economic icebergs and stabilize its deficit and debt problems. The U.S. economy would grow an average of 2.7 percent annually between 2010 and 2030, and the country’s share of Group of 20 gross domestic product would decline from about a third to about a quarter.

Dadush offered a second, bleaker picture, where breakup of the euro zone triggers a huge financial crisis that spreads to the United States. After several years of deep recession, the United States begins to expand but anemically. Under this forecast, U.S. growth would average just 1.5 percent through 2030. “Seen as a country on the down slide, the United States is both incapable of leading and disinclined to lead,” wrote Dadush about the more negative version.

A disturbing consensus emerged among the analysts that something closer to the pessimistic scenario should be the base line. Fred Kempe, president of the Atlantic Council, the think tank that hosted the meeting, sums up the views of these analysts and of a similar exercise last month by the World Economic Forum when he warns that the biggest national-security threat is “the danger of receding American influence on the world stage.”

My own view (I was asked to critique the presentations as an independent journalist) is that the key issue is how the United States adapts to adversity. That offers a slightly more encouraging picture: Relative to competitors, America still has a more adaptive financial system, stronger global corporations, a culture that can tap the talents of a diverse population and an unmatched military. The nation’s chronic weakness is its political system, which is approaching dysfunction. If the United States can elect better political leadership, it should be able to manage problems better than most competitors.

What other trends does the National Intelligence Council foresee in 2030? Burrows explained that the study will look at 15 or so “disruptive technologies” and their potential impact; it will examine governance and the growing gap between the pace of economic and political change and the ability of local, national and global governance to respond; and it will forecast likely conflicts — and assess ways that cyber, bio and other new weapons could empower individuals and small groups.

Here’s the most interesting footnote to this gloomy exercise. Burrows said that as he discusses his 2030 project with analysts around the world, he finds them much less downbeat about America’s prospects. “The Chinese are the first ones to say that we are too pessimistic about our future,” he reports, and Brazilian and Turkish analysts have said much the same thing.

Burrows noted that the nonpartisan report will be released after the 2012 presidential election. But the issue of America’s future will surely be at the heart of the coming campaign.