In Tuesday night’s State of the Union address, President Barack Obama touted the many ways that the economy has improved over the past year: lower unemployment, stronger economic growth, and lower gas prices. Wages are still stagnant but these improvements, particularly the lower gas prices, are boosting the president’s approval rating toward 50.

Republicans still aren’t willing to admit that the economy has taken significant steps over the past 12 months, but their economic attacks on the president have shifted. No longer do they cite the unemployment rate as an indicator of the failure of Obamanomics. The new statistic du jour is the labor force participation rate (LFPR). Republicans aren’t wrong to be concerned about this number. But the reason it’s trending downward predates Obama—and the party’s newfound focus on this stat is indicative of the political problem that an improving economy poses for the GOP.

The labor force participation rate is the number of people who are either employed or actively looking for work as a share of the population that could be working. Under Obama, the rate has declined from 65.7 percent in January 2009 to its current rate of 62.7 percent, its lowest point since the late 1970s. (It peaked at 67.3 percent in 2000.)

There are a few reasons why the LFPR has declined. First, the country is aging as baby boomers retire. An older country means a lower percent of the population will be in the labor force. This is a structural reason for the LFPR’s recent decline—it was going to happen regardless of the underlying economic conditions. That’s why many economists forecasted that the rate would slowly fall over time.

But then the financial crisis hit, sending the participation rate shooting down. Given the bleak economic conditions, many workers dropped out of the labor force altogether. These discouraged men and women would like a job, but think their prospects are too dim to even spend time looking for work. Younger Americans stayed in school, looking to bolster their resumes instead of entering the job market. The number of people who collect Social Security Disability Insurance has also risen substantially. Initially, at least, these were cyclical reasons for the LFPR’s decline—they were caused by the recession and were expected to turn around as the economy improved. Discouraged workers, for instance, were expected to reenter the labor market once the recovery took hold and their jobs prospects improved.