Politics always seems to get a bit off-kilter when the temperature goes up. But instead of the familiar silly-season stuff of years past -- made-up scandals and who-cares gossip -- the past two summers have been filled with vitriol. Last year we had town halls gone wild, fueled by the threat of death panels pulling the plug on Grandma. This year, us-vs.-them controversies are proliferating, linked by a surge in xenophobia. This is our summer of fear.

So far, the summer of fear has featured a charge, led by Newt Gingrich, Sarah Palin and former New York congressman Rick Lazio, to block the construction of the Cordoba House Islamic cultural center (which is to include a mosque) a few blocks from the site of the World Trade Center. Meanwhile, with frightening speed, we've gone from discussing the prospects for comprehensive immigration reform to watching congressional Republicans call for hearings to reconsider the 14th Amendment's guarantee of citizenship to anyone born in the United States.

Many liberals think they spy simple election-year opportunism. And of course, where there's an election, there's opportunism. But these prominent Republicans wouldn't be doing any of this if xenophobia weren't playing well politically.

Politicians are making hay out of the mosque only because public opinion seems to oppose it. They are reflecting, as well as stoking, a groundswell of public hostility toward outsiders. This hostility is not about the midterms; it is a consequence of the economic downturn, every bit as much as foreclosures and layoffs. When personal incomes stop growing, people become less broad-minded, and suspicion of foreigners and other ethnic groups grows. We have seen this time and again, in this country and in others.

Fear, in essence, begets fear. The loss of a job, or the worry that one might be lost, raises anxiety. This often plays out as increased suspicion of people who look different or come from different places. While times of robust growth and shared prosperity inspire feelings of interconnectedness and mutual gain, in times of worry, the picture quickly reverses. Views of the world turn zero-sum: If he wins, what do I lose? Any kind of change looks like decline -- the end of a "way of life."

One of the striking things about this particular shift is how quickly it has come about. Many expected racial tension during the 2008 presidential campaign, but it barely materialized. However, as unemployment and foreclosures have increased in the years since, so have trivial, race-based controversies, such as those surrounding the New Black Panther Party and Agriculture Department official Shirley Sherrod.

Similarly, the mosque controversy is not a continuation of the dynamics that started on Sept. 11, 2001, but a sharp reversal of course nine years on, one that's antithetical to the approach during the administration of President George W. Bush. Then, leading conservatives were careful to portray the U.S. response to the terrorist attacks as a targeted campaign against a minority group of murderous fanatics, not a broad cultural conflict with Islam. They appreciated that the latter approach would amount to a strategic and moral disaster.

The same abrupt slide toward xenophobia can be seen on immigration. As recently as 2004 and 2008, both parties nominated candidates who promised a path to citizenship for illegal immigrants. Today, President Obama is overseeing a skyrocketing rate of deportations, while Senate Minority Leader Mitch McConnell (R-Ky.) and Sen. John McCain (Ariz.) -- formerly the leading pro-immigration Republican -- are among those who want to hold hearings reconsidering the 14th Amendment. The shift has come so suddenly that the Republican Party hasn't even had time to amend its Web site, which as of this writing still touts "Republicans Passed the 14th Amendment" as one of the GOP's signature accomplishments.

At the same time, the number of Americans with doubts about the location of the president's birth appears to be on the rise. An Aug. 4 CNN poll found that only 42 percent of Americans are now sure he was born in the United States, way down from July 2009 polling that showed 77 percent of adults expressing certainty on this score. Despite clear evidence that he was born in Hawaii, Obama too is cast as an outsider.

Benjamin Friedman, an economist at Harvard whose 2005 book "The Moral Consequences of Economic Growth" argued that growth tends to foster liberal sentiments and open societies, whereas slowdowns undermine them, says this summer's events "are predictable consequences of this kind of sustained economic downturn."

"Manifestations like these have appeared in the U.S. at such times before," he told me, "most obviously in the 1880s and early 1890s," when a sustained period of economic stagnation coincided with the abandonment of the Reconstruction-era commitment to civil rights, the widespread adoption of anti-Chinese legislation and a nationwide wave of lynchings directed not only at blacks, but also Catholics and immigrants.

For progressives, this dynamic will take some getting used to. After the 2008 election, many liberals saw the recession as an opportunity for change. Rahm Emanuel's statement that "you never want a serious crisis to go to waste" was widely quoted, and comparisons to Franklin Roosevelt's first term proliferated.

In reality, though, recessions lead to illiberal populist nationalism, not progressive reform. If anti-immigrant sentiment was somewhat muted in the early '30s, it was because the doors from Europe had mostly been shut 10 years earlier, during another moment of economic dislocation -- the recession that followed the end of World War I. And, muted or not, anti-immigrant bias nonetheless inspired the Mexican Repatriation Program, which Herbert Hoover launched in 1929. That program would continue throughout the Depression, deporting hundreds of thousands of people of Mexican ancestry, many of them U.S. citizens.

What's often forgotten about the New Deal is that 1934-37 was the fastest four-year run of economic growth in American history, outside of World War II. In other words, it was the steep recovery from the Depression, not the Depression itself, that powered FDR's agenda forward.

The other high-water mark of liberalism, the Great Society -- including the creation of Medicare and Medicaid and the passage of the Civil Rights Act -- was similarly a child of rapid growth and prosperity, not of crisis.

The lesson is simple: The current controversies are ultimately byproducts of our economic morass. To really dispel the atmosphere of suspicion, what's needed are ideas about how to boost the economy to bring unemployment down and earnings up. Finding policies that do all this will not be easy, but it is the only way to turn the national mood around.

Those who support an open, pluralistic society won't get very far wading into these controversies one by one -- but that doesn't mean we shouldn't try. The economic roots of our summer of fear will hopefully prove transitory, but the rise in xenophobia may nonetheless inflict serious and permanent damage. A betrayal, even a fleeting one, of America's commitment to religious freedom could do lasting harm to the country's relationship with a billion Muslims around the world. And while altering the text of the 14th Amendment would be extremely difficult, and is therefore unlikely, the shouting matches now underway still stand to permanently scar our national identity.

Matthew Yglesias, a political blogger and fellow at the Center for American Progress Action Fund, is the author of "Heads in the Sand: How the Republicans Screw Up Foreign Policy and Foreign Policy Screws Up the Democrats."