On September 15 — the day Lehman Brothers filed for bankruptcy protection, Merrill Lynch was purchased by Bank of America, and AIG faltered — John McCain said in a speech that “the fundamentals of the American economy are strong.” Later in the day he revised that statement, explaining, “We’ve got to fix this economy, which the fundamentals of are at great risk right now . . .”

Then Barack Obama weighed in, saying of McCain: “He doesn’t get what’s happening between the mountain in Sedona where he lives and the corridors of Washington where he works. . . . Why else would he say, today, of all days — just a few hours ago — that the fundamentals of the economy are still strong?” The Obama camp then followed up with an ad titled “Fundamentals.”

Back up a minute. While McCain and Obama are busy drawing political battle lines around the “fundamentals of the American economy,” we’re still wondering what those fundamentals actually are. Can we get a definition?

Looking for answers, PaperTrail called the U.S. Labor Department’s Bureau of Labor Statistics. A spokesperson chuckled when asked whether BLS tracked the “fundamentals of the American economy,” and said that that is “not a term that the BLS uses.” The Bureau does, however, collect a set of data known as Principal Federal Economic Indicators, which are a big helping of alphabet soup:

Consumer Price Index, Employment Cost Index, The Employment Situation (which includes the unemployment rate and payroll employment), Producer Price Indexes, Productivity and Costs, Real Earnings, U.S. Import and Export Price Indexes.

Calling those indicators “strong” might be something of an overstatement. It’s a pretty mixed bag — some are up and some are down, on a month-to-month basis — but compared to this time last year, most are going the wrong direction. The Consumer Price Index, for example, is up 5.4 percent since this time last year, which means that people are paying more for the things they’re buying. Real average earnings rose 0.6 percent in August, but are down 2.5 percent in the last year.

Another phone call to the Commerce Department’s Bureau of Economic Analysis revealed that the BEA also does not have a statistic or set of statistics called “fundamentals of the economy.” A spokesperson suggested looking at the Gross Domestic Product data, which BEA compiles, and which is frequently used as a proxy for the nation’s economic well-being. The closest thing to fundamentals PaperTrail could find at BEA is the “Overview of the U.S. Economy: Perspective from the BEA Accounts,” which includes GDP, personal income, U.S. Balance of Payments, and several other indicators.

Again, the numbers are a mixed bag. Personal income is down. The current-account deficit increased last quarter, which means the United States owes more money than it did a few months ago. GDP was up last quarter by 3.3 percent, but has been on a rocky road.

As the economic upheaval continues and the rhetoric heats up, these statistics, fundamental or not, may be the best snapshot of how the nation is doing.